modesetting: unflip not possible when glamor is not set

This is fixing crashes of xfce when running under qemu

(cherry picked from commit 8836b9d2)
3 jobs for issue1195_fix_on_1.20_branch in 6 minutes and 7 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Docker Image
passed debian-testing #12299969

00:00:25

 
  Build And Test
passed autotools-build-and-test #12299970

00:05:41

passed meson-build-and-test #12299971

00:01:55