drm: use page flip event to signal flip completion

We don't actually know which frame number the flip will complete on, so
userspace needs a specific flip notification to tell it when the last flip
completed.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Kristian Høgsberg <krh@bitplanet.net>