[PATCH 09/11] modesetting: Implement PRIME syncing as a sink

[Alex Goins]

> I meant that no flip should be submitted either while there's no damage.

Yeah, but the source would need some way to tell the sink to flip after it
receives damage and finishes presenting.

Maybe I could make the sink register damage on the source's drawable directly,
then request a present/flip in response to that. It would preserve the semantics
of the sink explicitly requesting every present.

Or, in lieu of a another ABI function, just do the flip when receiving damage on
the sink's drawable.

Thanks,
Alex