drm/amd/amdgpu: skip locking delayed work if not initialized.

When init failed in early init stage, amdgpu_object has
not been initialized, so hasn't the ttm delayed queue functions.

Signed-off-by: YuBiao Wang <YuBiao.Wang@amd.com>
Reviewed-by: Emily.Deng <Emily.Deng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>