[non-normative] Propagate user_private when film grain enabled.

Cherry-pick of 881889ad456f758334f73791c7c97e0ea65433b3

BUG=aomedia:2163

Change-Id: I442cc87d214613d2a7ac16fdac27055638e74d93

av1/av1_dx_iface.c

diff --git a/av1/av1_dx_iface.c b/av1/av1_dx_iface.c
index 7e601ac..36ac9ce 100644
--- a/av1/av1_dx_iface.c
+++ b/av1/av1_dx_iface.c
@@ -621,6 +621,8 @@
     *grain_img_ptr = grain_img_buf;
   }
 
+  if (grain_img_buf) grain_img_buf->user_priv = img->user_priv;
+
   av1_add_film_grain(grain_params, img, grain_img_buf);
 
   return grain_img_buf;