drm/amdgpu: init umc v6_1 functions for vega20

init umc callback function for vega20 in sw early init phase

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Dennis Li <dennis.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>