drm/i915: Wrap -EIO send-vblank event for failed pageflip in spinlock

drm_send_vblank_event() demands that we hold the event spinlock whilst
calling it, so do so.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Fix the double lock as requested by Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -10084,8 +10084,11 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
 out_hang:
 		intel_crtc_wait_for_pending_flips(crtc);
 		ret = intel_pipe_set_base(crtc, crtc->x, crtc->y, fb);
-		if (ret == 0 && event)
+		if (ret == 0 && event) {
+			spin_lock_irqsave(&dev->event_lock, flags);
 			drm_send_vblank_event(dev, pipe, event);
+			spin_unlock_irqrestore(&dev->event_lock, flags);
+		}
 	}
 	return ret;
 }