compositor: don't try to repaint while offscreen or sleeping

weston_output_schedule_repaint() already checks the compositor state but
idle_repaint() is called asynchronously so the state may have changed.

Check the state again and abort if necessary.

Without this the DRM compositor might execute a modeset in
drm_output_start_repaint_loop() which should not happen while sleeping or
offscreen.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>