drm/scheduler: Set sched->thread to NULL on failure

In cases where the scheduler instance is used as a base object of another
driver object, it's not clear if the driver can call scheduler cleanup on the
fail path. So, Set the sched->thread to NULL, so that the driver can safely
call drm_sched_fini() during cleanup.

Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>