drm/amdgpu: fix null point error when rmmod amdgpu.

this bug happened when amdgpu load failed.

[   75.740951] BUG: unable to handle kernel paging request at 00000000000031c0
[   75.748167] IP: [<ffffffffa064a0e0>] amdgpu_fbdev_restore_mode+0x20/0x60 [amdgpu]
[   75.755774] PGD 0

[   75.759185] Oops: 0000 [#1] SMP
[   75.762408] Modules linked in: amdgpu(OE-) ttm(OE) drm_kms_helper(OE) drm(OE) i2c_algo_bit(E) fb_sys_fops(E) syscopyarea(E) sysfillrect(E) sysimgblt(E) rpcsec_gss_krb5(E) nfsv4(E) nfs(E) fscache(E) eeepc_wmi(E) asus_wmi(E) sparse_keymap(E) intel_rapl(E) snd_hda_codec_hdmi(E) snd_hda_codec_realtek(E) snd_hda_codec_generic(E) snd_hda_intel(E) snd_hda_codec(E) snd_hda_core(E) x86_pkg_temp_thermal(E) intel_powerclamp(E) snd_hwdep(E) snd_pcm(E) snd_seq_midi(E) coretemp(E) kvm_intel(E) snd_seq_midi_event(E) snd_rawmidi(E) kvm(E) snd_seq(E) joydev(E) snd_seq_device(E) snd_timer(E) irqbypass(E) crct10dif_pclmul(E) crc32_pclmul(E) mei_me(E) ghash_clmulni_intel(E) snd(E) aesni_intel(E) mei(E) soundcore(E) aes_x86_64(E) shpchp(E) serio_raw(E) lrw(E) acpi_pad(E) gf128mul(E) glue_helper(E) ablk_helper(E) mac_hid(E)
[   75.835574]  cryptd(E) parport_pc(E) ppdev(E) lp(E) nfsd(E) parport(E) auth_rpcgss(E) nfs_acl(E) lockd(E) grace(E) sunrpc(E) autofs4(E) hid_generic(E) usbhid(E) mxm_wmi(E) psmouse(E) e1000e(E) ptp(E) pps_core(E) ahci(E) libahci(E) wmi(E) video(E) i2c_hid(E) hid(E)
[   75.858489] CPU: 5 PID: 1603 Comm: rmmod Tainted: G           OE   4.9.0-custom #2
[   75.866183] Hardware name: System manufacturer System Product Name/Z170-A, BIOS 0901 08/31/2015
[   75.875050] task: ffff88045d1bbb80 task.stack: ffffc90002de4000
[   75.881094] RIP: 0010:[<ffffffffa064a0e0>]  [<ffffffffa064a0e0>] amdgpu_fbdev_restore_mode+0x20/0x60 [amdgpu]
[   75.891238] RSP: 0018:ffffc90002de7d48  EFLAGS: 00010286
[   75.896648] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000001
[   75.903933] RDX: 0000000000000000 RSI: ffff88045d1bbb80 RDI: 0000000000000286
[   75.911183] RBP: ffffc90002de7d50 R08: 0000000000000502 R09: 0000000000000004
[   75.918449] R10: 0000000000000000 R11: 0000000000000001 R12: ffff880464bf0000
[   75.925675] R13: ffffffffa0853000 R14: 0000000000000000 R15: 0000564e44f88210
[   75.932980] FS:  00007f13d5400700(0000) GS:ffff880476540000(0000) knlGS:0000000000000000
[   75.941238] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   75.947088] CR2: 00000000000031c0 CR3: 000000045fd0b000 CR4: 00000000003406e0
[   75.954332] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   75.961566] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   75.968834] Stack:
[   75.970881]  ffff880464bf0000 ffffc90002de7d60 ffffffffa0636592 ffffc90002de7d80
[   75.978454]  ffffffffa059015f ffff880464bf0000 ffff880464bf0000 ffffc90002de7da8
[   75.986076]  ffffffffa0595216 ffff880464bf0000 ffff880460f4d000 ffffffffa0853000
[   75.993692] Call Trace:
[   75.996177]  [<ffffffffa0636592>] amdgpu_driver_lastclose_kms+0x12/0x20 [amdgpu]
[   76.003700]  [<ffffffffa059015f>] drm_lastclose+0x2f/0xd0 [drm]
[   76.009777]  [<ffffffffa0595216>] drm_dev_unregister+0x16/0xd0 [drm]
[   76.016255]  [<ffffffffa0595944>] drm_put_dev+0x34/0x70 [drm]
[   76.022139]  [<ffffffffa062f365>] amdgpu_pci_remove+0x15/0x20 [amdgpu]
[   76.028800]  [<ffffffff81416499>] pci_device_remove+0x39/0xc0
[   76.034661]  [<ffffffff81531caa>] __device_release_driver+0x9a/0x140
[   76.041121]  [<ffffffff81531e58>] driver_detach+0xb8/0xc0
[   76.046575]  [<ffffffff81530c95>] bus_remove_driver+0x55/0xd0
[   76.052401]  [<ffffffff815325fc>] driver_unregister+0x2c/0x50
[   76.058244]  [<ffffffff81416289>] pci_unregister_driver+0x29/0x90
[   76.064466]  [<ffffffffa0596c5e>] drm_pci_exit+0x9e/0xb0 [drm]
[   76.070507]  [<ffffffffa0796d71>] amdgpu_exit+0x1c/0x32 [amdgpu]
[   76.076609]  [<ffffffff81104810>] SyS_delete_module+0x1a0/0x200
[   76.082627]  [<ffffffff810e2b1a>] ? rcu_eqs_enter.isra.36+0x4a/0x50
[   76.089001]  [<ffffffff8100392e>] do_syscall_64+0x6e/0x180
[   76.094583]  [<ffffffff817e1d2f>] entry_SYSCALL64_slow_path+0x25/0x25
[   76.101114] Code: 94 c0 c3 31 c0 5d c3 0f 1f 40 00 0f 1f 44 00 00 55 31 c0 48 89 e5 53 48 89 fb 48 c7 c7 1d 21 84 a0 e8 ab 77 b3 e0 e8 fc 8b d7 e0 <48> 8b bb c0 31 00 00 48 85 ff 74 09 e8 ff eb fc ff 85 c0 75 03
[   76.121432] RIP  [<ffffffffa064a0e0>] amdgpu_fbdev_restore_mode+0x20/0x60 [amdgpu]

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>