drm/sched: Check scheduler work queue before calling timeout handling

During an IGT GPU reset test we see again oops despite of
commit 0c8c901aaaebc9 (drm/sched: Check scheduler ready before calling
timeout handling).

It uses ready condition whether to call drm_sched_fault which unwind
the TDR leads to GPU reset.
However it looks the ready condition is overloaded with other meanings,
for example, for the following stack is related GPU reset :

0  gfx_v9_0_cp_gfx_start
1  gfx_v9_0_cp_gfx_resume
2  gfx_v9_0_cp_resume
3  gfx_v9_0_hw_init
4  gfx_v9_0_resume
5  amdgpu_device_ip_resume_phase2

does the following:
	/* start the ring */
	gfx_v9_0_cp_gfx_start(adev);
	ring->sched.ready = true;

The same approach is for other ASICs as well :
gfx_v8_0_cp_gfx_resume
gfx_v10_0_kiq_resume, etc...

As a result, our GPU reset test causes GPU fault which calls unconditionally gfx_v9_0_fault
and then drm_sched_fault. However now it depends on whether the interrupt service routine
drm_sched_fault is executed after gfx_v9_0_cp_gfx_start is completed which sets the ready
field of the scheduler to true even  for uninitialized schedulers and causes oops vs
no fault or when ISR  drm_sched_fault is completed prior  gfx_v9_0_cp_gfx_start and
NULL pointer dereference does not occur.

Use the field timeout_wq  to prevent oops for uninitialized schedulers.
The field could be initialized by the work queue of resetting the domain.

v1: Corrections to commit message (Luben)

Fixes: 11b3b9f461 ("drm/sched: Check scheduler ready before calling timeout handling")
Signed-off-by: Vitaly Prosyak <vitaly.prosyak@amd.com>
Link: https://lore.kernel.org/r/20230510135111.58631-1-vitaly.prosyak@amd.com
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>

drivers/gpu/drm/scheduler/sched_main.c

@@ -308,7 +308,7 @@ static void drm_sched_start_timeout(struct drm_gpu_scheduler *sched)
  */
 void drm_sched_fault(struct drm_gpu_scheduler *sched)
 {
-	if (sched->ready)
+	if (sched->timeout_wq)
 		mod_delayed_work(sched->timeout_wq, &sched->work_tdr, 0);
 }
 EXPORT_SYMBOL(drm_sched_fault);