modesetting: unflip not possible when glamor is not set

This is fixing crashes of xfce when running under qemu
6 jobs for master in 5 minutes and 32 seconds (queued for 10 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-buster #12295309

00:00:16

 
  Build And Test
passed autotools #12295310

00:05:16

passed meson #12295314

00:02:07

passed meson-noglamor #12295312

00:02:05

passed mingw-cross-build #12295313

00:00:45

failed meson #12295311

00:00:15