drm/scheduler: always put last_sched fence in entity_fini

Fix the potential memleak since scheduler main thread always
hold one last_sched fence.

Signed-off-by: Pixel Ding <Pixel.Ding@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>