drm/nouveau: Use drm_crtc_vblank_crtc()

Replace the open coded drm_crtc_vblank_crtc() with the real
thing.

Cc: Karol Herbst <[e-mail]>
Cc: Lyude Paul <[e-mail]>
Cc: Danilo Krummrich <[e-mail]>
Cc: [e-mail]
Signed-off-by: Ville Syrjälä <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Reviewed-by: Lyude Paul <[e-mail]>

drivers/gpu/drm/nouveau/nouveau_display.c

···
 nouveau_display_scanoutpos_head(struct drm_crtc *crtc, int *vpos, int *hpos,
 				ktime_t *stime, ktime_t *etime)
 {
-	struct drm_vblank_crtc *vblank = &crtc->dev->vblank[drm_crtc_index(crtc)];
+	struct drm_vblank_crtc *vblank = drm_crtc_vblank_crtc(crtc);
 	struct nvif_head *head = &nouveau_crtc(crtc)->head;
 	struct nvif_head_scanoutpos_v0 args;
 	int retry = 20;