drm/i915: Handle very early engine initialisation failure

If we fail during engine setup, we may leave some engines not yet setup.
During the error cleanup, we have to be careful not to try and use the
uninitialise engines before discarding them.

[   16.136152] RIP: 0010:__flush_work+0x198/0x1b0
[   16.136168] Code: ff ff 8b 0b 48 8b 53 08 83 e1 08 48 0f ba 2b 03 80 c9 f0 e9 63 ff ff ff 0f 0b 48 83 c4 48 44 89 f0 5b 5d 41 5c 41 5d 41 5e c3 <0f> 0b 45 31 f6 e9 62 ff ff ff 66 66 2e 0f 1f 84 00 00 00 00 00 0f
[   16.136186] RSP: 0018:ffffc900003bb928 EFLAGS: 00010246
[   16.136201] RAX: 0000000000000000 RBX: ffff88844f392168 RCX: 0000000000000000
[   16.136216] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88844f392168
[   16.136231] RBP: ffff88844f392130 R08: 0000000000000000 R09: 0000000000000001
[   16.136246] R10: ffff888441e31e40 R11: ffff88845e329c70 R12: ffff88844f796988
[   16.136261] R13: ffff888441e4fb80 R14: 0000000000000001 R15: ffff88844f790000
[   16.136388] FS:  00007fecbd208880(0000) GS:ffff88845e380000(0000) knlGS:0000000000000000
[   16.136405] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   16.136420] CR2: 00007ff3ce748f90 CR3: 0000000457a6a001 CR4: 00000000000606e0
[   16.136437] Call Trace:
[   16.136456]  ? try_to_del_timer_sync+0x3a/0x50
[   16.136529]  intel_wakeref_wait_for_idle+0x87/0xb0 [i915]
[   16.136606]  ? intel_engines_release+0x68/0xc0 [i915]
[   16.136680]  intel_engines_release+0x49/0xc0 [i915]
[   16.136757]  intel_gt_init+0x2f4/0x5e0 [i915]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200601072446.19548-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_engine_cs.c

@@ -414,12 +414,12 @@ void intel_engines_release(struct intel_gt *gt)
 
 	/* Decouple the backend; but keep the layout for late GPU resets */
 	for_each_engine(engine, gt, id) {
-		intel_wakeref_wait_for_idle(&engine->wakeref);
-		GEM_BUG_ON(intel_engine_pm_is_awake(engine));
-
 		if (!engine->release)
 			continue;
 
+		intel_wakeref_wait_for_idle(&engine->wakeref);
+		GEM_BUG_ON(intel_engine_pm_is_awake(engine));
+
 		engine->release(engine);
 		engine->release = NULL;