kms: Skip displays with 'non-desktop' property set

Detect displays marked as 'non-desktop' by the kernel and skip them when
creating the outputs. Mutter is not able to render images that are shown
properly on those devices anyway.

This avoids lighting up attached VR HMDs and showing the GDM login
screen between the eyes in a VR HMD instead of on the monitor.
6 jobs for !1393 with non-desktop in 14 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
failed #836332
check-commit-log

00:00:14

 
  Build
skipped #836333
build-mutter
skipped #836335
build-without-native-backend-and-wayland
skipped #836334
build-without-opengl-and-glx
 
  Test
skipped #836337
can-build-gnome-shell
skipped #836336
test-mutter
 
Name Stage Failure
failed
check-commit-log Review
Checking out 2c3bd40f as non-desktop...

Skipping Git submodules setup
Executing "step_script" stage of the job script
$ ./.gitlab-ci/check-commit-log.sh
From https://gitlab.gnome.org/GNOME/mutter
* branch master -> FETCH_HEAD
Commit 2c3bd40f needs a merge request or issue URL
ERROR: Job failed: exit code 1