drm/scheduler: use job count instead of peek

The spsc_queue_peek function is accessing queue->head which belongs to
the consumer thread and shouldn't be accessed by the producer

This is fixing a rare race condition when destroying entities.

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