drm/sched: Avoid infinite waits in the drm_sched_entity_destroy() path

If we don't initialize the entity to idle and the entity is never
scheduled before being destroyed we end up with an infinite wait in the
destroy path.

v2:
- Add Steven's R-b

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/393486/