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

In the Linux kernel, the following vulnerability has been resolved:

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)