Allow to enable atomic in modesetting DDX

The linux kernel blacklisted atomic for the modesetting DDX to deal with
problems with old Xorg versions which didn't have it disabled by default.
Ever since, it wasn't possible to enable atomic in modesetting at all
anymore.

This change doesn't enable atomic by default, but it allows a user to
enabled it again.

Signed-off-by: Daniel Abrecht <public@danielabrecht.ch>
4 jobs for enable-atomic in 5 minutes and 57 seconds
latest
Status Name Job ID Coverage
  Docker Image
passed debian-testing #5153697

00:00:27

 
  Build And Test
passed autotools #5153698

00:05:29

passed meson #5153699

00:00:59

passed meson-noglamor #5153700

00:01:18