compositor: add dpms and backlight support

DPMS kicks in only when wscreensaver is launched, in the moment that shell
call lock() for the second time. Backlight control internals are managed by
libbacklight:

	http://cgit.freedesktop.org/~vignatti/libbacklight/

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>
diff --git a/src/compositor-wayland.c b/src/compositor-wayland.c
index a42e76f..f8b5a32 100644
--- a/src/compositor-wayland.c
+++ b/src/compositor-wayland.c
@@ -435,6 +435,8 @@
 	output->base.repaint = wayland_output_repaint;
 	output->base.destroy = wayland_output_destroy;
 	output->base.assign_planes = NULL;
+	output->base.set_backlight = NULL;
+	output->base.set_dpms = NULL;
 
 	wl_list_insert(c->base.output_list.prev, &output->base.link);