drm/amdgpu: add gmc ip block for dimgrey_cavefish

Enable gmc block for dimgrey_cavefish, same as sienna_cichlid.

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