Backported X.org commits:

 commit b380f3ac51f40ffefcde7d3db5c4c149f274246d
 Author: Michel Dänzer <michel.daenzer@amd.com>
 Date:   Tue Aug 2 17:53:01 2016 +0900

    dix: Pass ClientPtr to FlushCallback

    This change has two effects:

    1. Only calls FlushCallbacks when we're actually flushing data to a
       client. The unnecessary FlushCallback calls could cause significant
       performance degradation with compositing, which is significantly
       reduced even without any driver changes.

    2. By passing the ClientPtr to FlushCallbacks, drivers can completely
       eliminate unnecessary flushing of GPU commands by keeping track of
       whether we're flushing any XDamageNotify events to the client for
       which the corresponding rendering commands haven't been flushed to
       the GPU yet.
Reviewed-by: Adam Jackson <ajax@redha.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>

 commit c65f610e12f9df168d5639534ed3c2bd40afffc8
 Author: Kristian Høgsberg <krh@bitplanet.net>
 Date:   Thu Jul 29 18:52:35 2010 -0400

    Always call the flush callback chain when we flush client buffers

    We were missing the callback in a couple of places.  Drivers may use
    the flush callback to submit batched up rendering before events (for
    example, damage events) are sent out, to ensure that the rendering
    has been queued when the client receives the event.
Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Keith Packard <keithp@keithp.com>
Backported-to-NX-by: Mike Gabriel <mike.gabriel@das-netzwerkteam.de>