drm/sched: select new rq even if there is only one v3

This is necessary when changing priorities of an entity.

v2: test the sched_list instead of num_sched.
v3: set the sched_list to NULL when there is only one entry

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Sonny Jiang <sonny.jiang@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210305125155.2312-1-christian.koenig@amd.com