drm: rockchip: use crtc helper drm_crtc_from_index()

Function rockchip_crtc_from_pipe() does the exactly same thing as what
crtc helper drm_crtc_from_index() provides.  Use the helper to save
some code.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483015290-16660-4-git-send-email-shawnguo@kernel.org

drivers/gpu/drm/rockchip/rockchip_drm_drv.c

@@ -99,24 +99,11 @@ void rockchip_unregister_crtc_funcs(struct drm_crtc *crtc)
 	priv->crtc_funcs[pipe] = NULL;
 }
 
-static struct drm_crtc *rockchip_crtc_from_pipe(struct drm_device *drm,
-						int pipe)
-{
-	struct drm_crtc *crtc;
-	int i = 0;
-
-	list_for_each_entry(crtc, &drm->mode_config.crtc_list, head)
-		if (i++ == pipe)
-			return crtc;
-
-	return NULL;
-}
-
 static int rockchip_drm_crtc_enable_vblank(struct drm_device *dev,
 					   unsigned int pipe)
 {
 	struct rockchip_drm_private *priv = dev->dev_private;
-	struct drm_crtc *crtc = rockchip_crtc_from_pipe(dev, pipe);
+	struct drm_crtc *crtc = drm_crtc_from_index(dev, pipe);
 
 	if (crtc && priv->crtc_funcs[pipe] &&
 	    priv->crtc_funcs[pipe]->enable_vblank)
@@ -129,7 +116,7 @@ static void rockchip_drm_crtc_disable_vblank(struct drm_device *dev,
 					     unsigned int pipe)
 {
 	struct rockchip_drm_private *priv = dev->dev_private;
-	struct drm_crtc *crtc = rockchip_crtc_from_pipe(dev, pipe);
+	struct drm_crtc *crtc = drm_crtc_from_index(dev, pipe);
 
 	if (crtc && priv->crtc_funcs[pipe] &&
 	    priv->crtc_funcs[pipe]->enable_vblank)