drm/i915/guc: Protect against no desc-pool on premature shutdown

drm/i915/guc: Protect against no desc-pool on premature shutdown

Hopefully the final hack to get guc fault-injection happy before we can
clean it up again, starting from a known good baseline...

[  383.017530] BUG: unable to handle kernel NULL pointer dereference at 00000000000000a0
[  383.017556] Oops: 0000 [#1] PREEMPT SMP PTI
[  383.017566] CPU: 7 PID: 4725 Comm: drv_module_relo Tainted: G     U            4.18.0-rc4-CI-CI_DRM_4485+ #1
[  383.017581] Hardware name: Micro-Star International Co., Ltd. MS-7B54/Z370M MORTAR (MS-7B54), BIOS 1.10 12/28/2017
[  383.017664] RIP: 0010:guc_stage_desc_pool_destroy+0x17/0xe0 [i915]
[  383.017674] Code: 59 a0 c6 05 02 59 18 00 01 e8 5e 01 c3 e0 eb b1 0f 1f 00 53 48 89 fb 48 81 c7 90 02 00 00 e8 60 64 45 e1 48 8b 83 80 02 00 00 <48> 8b 80 a0 00 00 00 48 8b 90 68 02 00 00 48 83 ea 01 48 81 fa ff
[  383.017771] RSP: 0018:ffffc900004bbdd0 EFLAGS: 00010282
[  383.017782] RAX: 0000000000000000 RBX: ffff88012ff41300 RCX: 0000000000000000
[  383.017794] RDX: 0000000000000000 RSI: ffffc900004bbd80 RDI: 0000000000000000
[  383.017805] RBP: ffff88012ff40000 R08: 00000000d876ee11 R09: 0000000000000000
[  383.017817] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88012ff47770
[  383.017828] R13: ffff88012ff40068 R14: ffff880264392ef8 R15: ffffffffa0639950
[  383.017840] FS:  00007fb9c18c8980(0000) GS:ffff8802663c0000(0000) knlGS:0000000000000000
[  383.017853] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  383.017864] CR2: 00000000000000a0 CR3: 00000001df6cc003 CR4: 00000000003606e0
[  383.017875] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  383.017887] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  383.017898] Call Trace:
[  383.017962]  intel_uc_fini+0x34/0xd0 [i915]
[  383.018020]  i915_gem_fini+0x5c/0x100 [i915]
[  383.018093]  i915_driver_unload+0xd2/0x110 [i915]
[  383.018150]  i915_pci_remove+0x10/0x20 [i915]
[  383.018165]  pci_device_remove+0x36/0xb0
[  383.018179]  device_release_driver_internal+0x185/0x250
[  383.018193]  driver_detach+0x35/0x70
[  383.018205]  bus_remove_driver+0x53/0xd0
[  383.018217]  pci_unregister_driver+0x25/0xa0
[  383.018232]  __se_sys_delete_module+0x162/0x210
[  383.018245]  ? do_syscall_64+0xd/0x190
[  383.018257]  do_syscall_64+0x55/0x190
[  383.018270]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  383.018282] RIP: 0033:0x7fb9c0f7c1b7
[  383.018290] Code: 73 01 c3 48 8b 0d d1 8c 2c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 b8 b0 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d a1 8c 2c 00 f7 d8 64 89 01 48
[  383.018408] RSP: 002b:00007fffa01c2aa8 EFLAGS: 00000206 ORIG_RAX: 00000000000000b0
[  383.018425] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fb9c0f7c1b7
[  383.018440] RDX: 0000000000000000 RSI: 0000000000000800 RDI: 0000560b96856d48
[  383.018454] RBP: 0000560b96856ce0 R08: 0000560b96856d4c R09: 00007fffa01c2ae8
[  383.018468] R10: 00007fffa01c1aa4 R11: 0000000000000206 R12: 0000560b954f7470

Testcase: igt/drv_module_reload/basic-reload-inject
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180713172658.14070-1-chris@chris-wilson.co.uk