drm/vc4: Use the helper to retrieve vc4_dev when needed

We have a helper to retrieve the vc4_dev structure from the drm_device one
when needed, so let's use it consistently.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201029190104.2181730-4-maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_kms.c

@@ -51,7 +51,7 @@ static struct vc4_ctm_state *vc4_get_ctm_state(struct drm_atomic_state *state,
 					       struct drm_private_obj *manager)
 {
 	struct drm_device *dev = state->dev;
-	struct vc4_dev *vc4 = dev->dev_private;
+	struct vc4_dev *vc4 = to_vc4_dev(dev);
 	struct drm_private_state *priv_state;
 	int ret;