drm/radeon/kms: pageflipping cleanup for avivo+

Avoid touching the flip setup regs while
acceleration is running.  Set them at modeset
rather than during pageflip.  Touching these
regs while acceleration is active caused hangs
on pre-avivo chips.  These chips do not seem
to be affected, but better safe than sorry,
plus it avoids repeatedly reprogramming the
regs every flip.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>