drm/radeon/kms: allow r500 US_FORMAT regs in the CS checker

add to the 2.8 bump for pageflip

The purpose of these regs is to work around a TX hw bug in R520.

Signed-off-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>