drm/amdgpu: add gfx ip block for dimgrey_cavefish

Enable gfx block for dimgrey_cavefish, same as navy_flounder.

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>