drm/amdgpu: use error label to handle exceptional case

Use error label to decrease global item unref and make codes more
readable.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>