drm/amdgpu: add missing NULL check

bo->tbo.resource can easily be NULL here.

Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2902


Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
CC: stable@vger.kernel.org