drm/amdgpu/pm: Remove gpu_od if it's an empty directory

gpu_od should be removed if it's an empty directory

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reported-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org