drm/amdgpu: move umc late init from gmc to umc block

umc late init is umc specific, it's more suitable to be put in umc block

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>