drm/amdgpu: fix null pointer panic with direct fw loading on gpu reset

When system uses fw direct loading, then psp context structure won't be
initiliazed. And it is also unable to execute mode reset.

[  434.601474] amdgpu 0000:0c:00.0: GPU reset begin!
[  434.694326] amdgpu 0000:0c:00.0: GPU reset
[  434.743152] BUG: unable to handle kernel NULL pointer dereference at
0000000000000058
[  434.838474] IP: psp_gpu_reset+0xc/0x30 [amdgpu]
[  434.893532] PGD 406ed9067
[  434.893533] P4D 406ed9067
[  434.926376] PUD 400b46067
[  434.959217] PMD 0
[  435.033379] Oops: 0000 [#1] SMP
[  435.072573] Modules linked in: amdgpu(OE) chash(OE) gpu_sched(OE) ttm(OE)
drm_kms_helper(OE) drm(OE) fb_sys_fops syscopyarea sysfillrect sysimgblt
rpcsec_gss_krb5 auth_rpcgss nfsv4 nfs lockd grace fscache snd_hda_codec_realtek
snd_hda_codec_generic snd_hda_codec_hdmi snd_hda_intel snd_hda_codec
snd_hda_core snd_hwdep snd_pcm edac_mce_amd snd_seq_midi snd_seq_midi_event
kvm_amd snd_rawmidi kvm irqbypass crct10dif_pclmul crc32_pclmul snd_seq
ghash_clmulni_intel snd_seq_device pcbc snd_timer eeepc_wmi aesni_intel snd
asus_wmi aes_x86_64 sparse_keymap crypto_simd glue_helper joydev soundcore
wmi_bmof cryptd video i2c_piix4 shpchp 8250_dw i2c_designware_platform mac_hid
i2c_designware_core sunrpc parport_pc ppdev lp parport autofs4 hid_generic igb
usbhid dca ptp mxm_wmi pps_core ahci hid i2c_algo_bit
[  435.931754]  libahci wmi

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>