drm/amdgpu: move mmhub_ras_if from gmc to mmhub block

mmhub_ras_if is relevant to mmhub

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>