drm/scheduler: add new function to get least loaded sched v2

The function selects the run queue from the rq_list with the
least load. The load is decided by the number of jobs in a
scheduler.

v2: avoid using atomic read twice consecutively, instead store
    it locally

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>