drm/amdgpu: only allow error injection to UMC IP block

error injection to other IP blocks (except UMC) will be enabled
until RAS feature stablize on those IP blocks

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>