drm/ttm: don't oops if no invalidate_caches()

A few of the simpler TTM drivers (cirrus, ast, mgag200) do not implement
this function.  Yet can end up somehow with an evicted bo:

  BUG: unable to handle kernel NULL pointer dereference at           (null)
  IP: [<          (null)>]           (null)
  PGD 16e761067 PUD 16e6cf067 PMD 0
  Oops: 0010 [#1] SMP
  Modules linked in: bnep bluetooth rfkill fuse ip6t_rpfilter ip6t_REJECT ipt_REJECT xt_conntrack ebtable_nat ebtable_broute bridge stp llc ebtable_filter ebtables ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_mangle ip6table_security ip6table_raw ip6table_filter ip6_tables iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_mangle iptable_security iptable_raw iptable_filter ip_tables sg btrfs zlib_deflate raid6_pq xor dm_queue_length iTCO_wdt iTCO_vendor_support coretemp kvm dcdbas dm_service_time microcode serio_raw pcspkr lpc_ich mfd_core i7core_edac edac_core ses enclosure ipmi_si ipmi_msghandler shpchp acpi_power_meter mperf nfsd auth_rpcgss nfs_acl lockd uinput sunrpc dm_multipath xfs libcrc32c ata_generic pata_acpi sr_mod cdrom
   sd_mod usb_storage mgag200 syscopyarea sysfillrect sysimgblt i2c_algo_bit lpfc drm_kms_helper ttm crc32c_intel ata_piix bfa drm ixgbe libata i2c_core mdio crc_t10dif ptp crct10dif_common pps_core scsi_transport_fc dca scsi_tgt megaraid_sas bnx2 dm_mirror dm_region_hash dm_log dm_mod
  CPU: 16 PID: 2572 Comm: X Not tainted 3.10.0-86.el7.x86_64 #1
  Hardware name: Dell Inc. PowerEdge R810/0H235N, BIOS 0.3.0 11/14/2009
  task: ffff8801799dabc0 ti: ffff88016c884000 task.ti: ffff88016c884000
  RIP: 0010:[<0000000000000000>]  [<          (null)>]           (null)
  RSP: 0018:ffff88016c885ad8  EFLAGS: 00010202
  RAX: ffffffffa04e94c0 RBX: ffff880178937a20 RCX: 0000000000000000
  RDX: 0000000000000000 RSI: 0000000000240004 RDI: ffff880178937a00
  RBP: ffff88016c885b60 R08: 00000000000171a0 R09: ffff88007cf171a0
  R10: ffffea0005842540 R11: ffffffff810487b9 R12: ffff880178937b30
  R13: ffff880178937a00 R14: ffff88016c885b78 R15: ffff880179929400
  FS:  00007f81ba2ef980(0000) GS:ffff88007cf00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000000 CR3: 000000016e763000 CR4: 00000000000007e0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
  Stack:
   ffffffffa0306fae ffff8801799295c0 0000000000260004 0000000000000001
   ffff88016c885b60 ffffffffa0307669 00ff88007cf17738 ffff88017cf17700
   ffff880178937a00 ffff880100000000 ffff880100000000 0000000079929400
  Call Trace:
   [<ffffffffa0306fae>] ? ttm_bo_handle_move_mem+0x54e/0x5b0 [ttm]
   [<ffffffffa0307669>] ? ttm_bo_mem_space+0x169/0x340 [ttm]
   [<ffffffffa0307bd7>] ttm_bo_move_buffer+0x117/0x130 [ttm]
   [<ffffffff81130001>] ? perf_event_init_context+0x141/0x220
   [<ffffffffa0307cb1>] ttm_bo_validate+0xc1/0x130 [ttm]
   [<ffffffffa04e7377>] mgag200_bo_pin+0x87/0xc0 [mgag200]
   [<ffffffffa04e56c4>] mga_crtc_cursor_set+0x474/0xbb0 [mgag200]
   [<ffffffff811971d2>] ? __mem_cgroup_commit_charge+0x152/0x3b0
   [<ffffffff815c4182>] ? mutex_lock+0x12/0x2f
   [<ffffffffa0201433>] drm_mode_cursor_common+0x123/0x170 [drm]
   [<ffffffffa0205231>] drm_mode_cursor_ioctl+0x41/0x50 [drm]
   [<ffffffffa01f5ca2>] drm_ioctl+0x502/0x630 [drm]
   [<ffffffff815cbab4>] ? __do_page_fault+0x1f4/0x510
   [<ffffffff8101cb68>] ? __restore_xstate_sig+0x218/0x4f0
   [<ffffffff811b4445>] do_vfs_ioctl+0x2e5/0x4d0
   [<ffffffff8124488e>] ? file_has_perm+0x8e/0xa0
   [<ffffffff811b46b1>] SyS_ioctl+0x81/0xa0
   [<ffffffff815d05d9>] system_call_fastpath+0x16/0x1b
  Code:  Bad RIP value.
  RIP  [<          (null)>]           (null)
   RSP <ffff88016c885ad8>
  CR2: 0000000000000000

Signed-off-by: Rob Clark <rclark@redhat.com>
Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Cc: stable@vger.kernel.org