compositor-fbdev: fix finish_frame_timer leak

The timer was never removed anywhere. Remove it in disable() to match
what happens in enable().

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

libweston/compositor-fbdev.c

@@ -486,6 +486,9 @@ fbdev_output_disable(struct weston_output *base)
 	if (!base->enabled)
 		return 0;
 
+	wl_event_source_remove(output->finish_frame_timer);
+	output->finish_frame_timer = NULL;
+
 	pixman_renderer_output_destroy(&output->base);
 	fbdev_frame_buffer_unmap(output);