backend-rdp: Use weston_output_finish_frame_from_timer()

Replace the finish frame handler logic with a call to the
weston_output_finish_frame_from_timer() helper function.

This makes finish_frame_handler() return more exact timestamps
calculated from the previous frame time if the timer callback
was not delayed too much.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

libweston/backend-rdp/rdp.c

@@ -308,10 +308,8 @@ static int
 finish_frame_handler(void *data)
 {
 	struct rdp_output *output = data;
-	struct timespec ts;
 
-	weston_compositor_read_presentation_clock(output->base.compositor, &ts);
-	weston_output_finish_frame(&output->base, &ts, 0);
+	weston_output_finish_frame_from_timer(&output->base);
 
 	return 1;
 }