drm/amdgpu: remove useless condition in amdgpu_job_stop_all_jobs_on_sched()

Local variable 'rq' is initialized by an address
of field of drm_sched_job, so it does not make
sense to compare 'rq' with NULL.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Andrey Strachuk <strochuk@ispras.ru>
Fixes: 7c6e68c7 ("drm/amdgpu: Avoid HW GPU reset for RAS.")
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>