drm/amdgpu: add gmc cg support for dimgrey_cavefish

The athub version for dimgrey_cavefish is v2.1.

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>