Fix present_pixmap when using present_notify_msc

Calling present_notify_msc could cancel a pending pixmap presentation.

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
(cherry picked from commit 9bc01dfc7070a40f5948588895b3a11dd1636d0e)
Signed-off-by: Julien Cristau <jcristau@debian.org>