modesetting: Add support for headless display

In some Virtualization environments, a CRTC may not be available but a
graphics device may still be present (/dev/dri/card0). In such
situations, the modesetting DDX should still load Glamor (and DRI) and
work in a "Headless" fashion.

Present's screen flip mode will handle the vblanks (and flips) in these
situations.

Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
4 jobs for headless in 6 minutes and 19 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Docker Image
passed debian-testing #3928184

00:00:25

 
  Build And Test
passed autotools #3928185

00:05:53

passed meson #3928186

00:02:23

passed meson-noglamor #3928187

00:02:08