drm/scheduler: add a pointer to scheduler in the rq

This patch is in preparation for a better load balancing in
scheduler. It allows us to associate entities with the
run queues instead of binding them to a scheduler.

Signed-off-by: Nayan Deshmukh <nayan26deshmukh@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>