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.
Possible fix for #1367.