Based on xf86-video-ati

commit 9a1afbf61fbb2827c86bd86d295fa0848980d60b
Author: Michel Dänzer <michel.daenzer@amd.com>
Date:   Mon Jul 11 12:22:09 2016 +0900

    Use EventCallback to avoid flushing every time in the FlushCallback

reports seeing an improvement in reducing flushes at the expense of
checking every event for a DamageNotifyEvent. Since we also mix
rendering with SHM buffers, we have a more diverse set of conditions
under which to flush - but maybe we will see enough of a win for DRI to
merit. So far seeing improvement of ~20% for series of small operations
under the compositor without seeing any regressions, should benefit
composited desktop users. The biggest danger here is missed flushes.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>