drm: rcar-du: Clear handled event pointer in CRTC state

The atomic commit helper requires drivers to clear the event pointer
stored in the CRTC state when the event is handled. In preparation to
using the helper, fix the driver.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

drivers/gpu/drm/rcar-du/rcar_du_crtc.c

@@ -501,16 +501,16 @@ static void rcar_du_crtc_disable(struct drm_crtc *crtc)
 static void rcar_du_crtc_atomic_begin(struct drm_crtc *crtc,
 				      struct drm_crtc_state *old_crtc_state)
 {
-	struct drm_pending_vblank_event *event = crtc->state->event;
 	struct rcar_du_crtc *rcrtc = to_rcar_crtc(crtc);
 	struct drm_device *dev = rcrtc->crtc.dev;
 	unsigned long flags;
 
-	if (event) {
+	if (crtc->state->event) {
 		WARN_ON(drm_crtc_vblank_get(crtc) != 0);
 
 		spin_lock_irqsave(&dev->event_lock, flags);
-		rcrtc->event = event;
+		rcrtc->event = crtc->state->event;
+		crtc->state->event = NULL;
 		spin_unlock_irqrestore(&dev->event_lock, flags);
 	}