drm/amd/amdgpu: fix null pointer in runtime pm

fix the null pointer issue when runtime pm is triggered.

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org