drm/amdgpu: add amdgpu_mmhub_funcs definition

add amdgpu_mmhub_funcs definition and initialize it,
prepare for mmhub ras enablement

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>