Xext/shm: Detach SHM segment after Pixmap is released

The GPU may still have a reference to the SHM segment which would only
be finally released when the Pixmap is destroy. So we can only detach
the SHM segment (and thereby making the memory unaccessible) after the
backend has had a chance to flush any remaining references.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=85058
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reported-and-tested-by: gedgon@gmail.com
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
(cherry picked from commit 9b29fa957a397664463c7c78fbcc2f34d1993271)
Signed-off-by: Julien Cristau <jcristau@debian.org>
Bump to 1.16.1.901