UBUNTU: (pre-stable) drm/radeon/kms: fix crtc vblank update for r600

In 2.6.32.2 r600 had no IRQ support, however the patch in
500b7587

 to fix vblanks on avivo
cards, needs irqs.

So check for an R600 card and avoid this path if so.

This is a stable only patch for 2.6.32.2 as 2.6.33 has IRQs for r600.

Signed-off-by: Dave Airlie <airlied@redhat.com>