drm/vblank: use drm_crtc_vblank_crtc() in workers

We have drm_crtc_vblank_crtc() to get the struct drm_vblank_crtc pointer
for a crtc. Use it instead of poking at dev->vblank[] directly.

Reviewed-by: Thomas Zimmermann <[e-mail]>
Link: https://patch.msgid.link/f04[e-mail]
Signed-off-by: Jani Nikula <[e-mail]>

drivers/gpu/drm/drm_vblank_work.c

···
 void drm_vblank_work_flush_all(struct drm_crtc *crtc)
 {
 	struct drm_device *dev = crtc->dev;
-	struct drm_vblank_crtc *vblank = &dev->vblank[drm_crtc_index(crtc)];
+	struct drm_vblank_crtc *vblank = drm_crtc_vblank_crtc(crtc);
 
 	spin_lock_irq(&dev->event_lock);
 	wait_event_lock_irq(vblank->work_wait_queue,