drm/i915/vblank: use drm_crtc_vblank_crtc() instead of open-coding

There's a helper for drm->vblank[drm_crtc_index(crtc)], use it.

Reviewed-by: Rodrigo Vivi <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/781[e-mail]
Signed-off-by: Jani Nikula <[e-mail]>

drivers/gpu/drm/i915/display/intel_vblank.c

···
 u32 i915_get_vblank_counter(struct drm_crtc *crtc)
 {
 	struct drm_i915_private *dev_priv = to_i915(crtc->dev);
-	struct drm_vblank_crtc *vblank = &dev_priv->drm.vblank[drm_crtc_index(crtc)];
+	struct drm_vblank_crtc *vblank = drm_crtc_vblank_crtc(crtc);
 	const struct drm_display_mode *mode = &vblank->hwmode;
 	enum pipe pipe = to_intel_crtc(crtc)->pipe;
 	u32 pixel, vbl_start, hsync_start, htotal;
···
 u32 g4x_get_vblank_counter(struct drm_crtc *crtc)
 {
 	struct drm_i915_private *dev_priv = to_i915(crtc->dev);
-	struct drm_vblank_crtc *vblank = &dev_priv->drm.vblank[drm_crtc_index(crtc)];
+	struct drm_vblank_crtc *vblank = drm_crtc_vblank_crtc(crtc);
 	enum pipe pipe = to_intel_crtc(crtc)->pipe;
 
 	if (!vblank->max_vblank_count)