drm/radeon/kms: make the mac rv630 quirk generic

Seems some other boards do this as well.

Reported-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@gmail.com>