drm/scheduler: fix drm_sched_get_cleanup_job

We are racing to initialize sched->thread here, just always check the
current thread.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Kent Russell <kent.russell@amd.com>
Link: https://patchwork.freedesktop.org/patch/361303/