drm/sched: Use completion to wait for sched->thread idle v2.

Removes thread park/unpark hack from drm_sched_entity_fini and
by this fixes reactivation of scheduler thread while the thread
is supposed to be stopped.

v2: Per sched entity completion.

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