Make xf86CompatOutput() return NULL when there are no privates

Some drivers (mach64 w/o DRI for instance) don't initialize privates.

Signed-off-by: Matthieu Herrb <matthieu.herrb@laas.fr>
4 jobs for mherrb-fix-compat-output in 2 minutes and 45 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-buster #15450135

00:00:16

 
  Build And Test
passed meson #15450136

00:02:29

passed meson-noglamor #15450137

00:01:59

passed mingw-cross-build #15450138

00:00:46