drm/scheduler: add NULL pointer check for run queue (v2)

To check rq pointer before adding entity into it.
That avoids NULL pointer access in some case.

v2: move the check to caller

Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>