drm/scheduler: add a list of run queues to the entity

These are the potential run queues on which the jobs from this
entity can be scheduled. We will use this to do load balancing.

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