drm/amdgpu: fix mutex list null pointer reference

Fix NULL pointer reference.

Signed-off-by: Pixel Ding <Pixel.Ding@amd.com>
Signed-off-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>