modesetting: Allow enabling atomic modeset by option

Most significant issues have been fixed so let's allow overriding kernel
disallowing us to use atomic modeset.

Signed-off-by: Povilas Kanapickas <povilas@radix.lt>
4 jobs for modeset-atomic in 4 minutes and 42 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Docker Image
passed #8742213
debian-buster

00:00:10

 
  Build And Test
passed #8742214
autotools

00:04:32

passed #8742215
meson

00:01:47

passed #8742216
meson-noglamor

00:01:36