backend/drm: add wlr_drm_backend_monitor

This helper is responsible for listening for new DRM devices and
create new child DRM backends as necessary.

backend/drm/meson.build

@@ -4,6 +4,7 @@ wlr_files += files(
 	'cvt.c',
 	'drm.c',
 	'legacy.c',
+	'monitor.c',
 	'properties.c',
 	'renderer.c',
 	'util.c',