drm/sched: Avoid job cleanup if sched thread is parked.

When the sched thread is parked we assume ring_mirror_list is
not accessed from here.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>