drm/radeon/kms: fix mac g5 quirk

Apple uses the same subsystem pci ids for lots of
hardware much of which is wired up differently.  In
this case, the G5 imac and the G5 tower.

Only apply the quirk configuration to G5 towers.

Reported-by: Joachim Henke <j-o@users.sourceforge.net>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: Joachim Henke <j-o@users.sourceforge.net>
Cc: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>