Propagate img->fb_priv when film grain enabled.

This is analogous to Tom Finegan's change to propagate img->user_priv in
commit 881889ad456f758334f73791c7c97e0ea65433b3:
https://aomedia-review.googlesource.com/c/aom/+/71021/2

BUG=aomedia:1255

Change-Id: I1d87c31254633813a38cf79ebc02068fab2456ee
diff --git a/av1/av1_dx_iface.c b/av1/av1_dx_iface.c
index d453dbb..5b4edfb 100644
--- a/av1/av1_dx_iface.c
+++ b/av1/av1_dx_iface.c
@@ -655,6 +655,7 @@
 
   if (grain_img_buf) {
     grain_img_buf->user_priv = img->user_priv;
+    grain_img_buf->fb_priv = img->fb_priv;
     if (av1_add_film_grain(grain_params, img, grain_img_buf)) {
       aom_img_free(grain_img_buf);
       grain_img_buf = NULL;