This will prevent things from falling over if the user frees the flip
buffer before we complete the flip, since we'll hold an internal
reference.
Reported-by: NKristian Høgsberg <krh@bitplanet.net>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NEric Anholt <eric@anholt.net>