drm: disallow legacy sg ioctls for modesetting drivers

Only the radeon/r128/ati ums drivers use this. Furthermore the cleanup
was already only done for UMS drivers. Also a quick check of the ATI
ddx git history shows that only the UMS code ever used this facility.

So we can safely disallow these pair of ioctls for modesetting
drivers.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>