drm/amdgpu: add list empty check to avoid null pointer issue

Add list empty check to avoid null pointer issues in some corner cases.
- list_for_each_entry_safe()

Signed-off-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>