drm/radeon: add pageflip hooks for fusion

Looks like these got passed over with both being merged at the same
time but not quite meeting in the middle.

should fix: https://bugs.freedesktop.org/show_bug.cgi?id=34137


along with Michael's phoronix article.

Reported-by: Chi-Thanh Christopher Nguyen
Article-written-by: Michael Larabel @ phoronix
Signed-off-by: Dave Airlie <airlied@redhat.com>