Simplify context identification for coding ref frames

This patch simply aggregates the checking on the counts of certain
reference frames in the neighboring above and left blocks. It does
not incur any coding performance change.

Change-Id: I59a962ba95e7ab16731ce97371ec5709a582a0ba
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 443a856..8fa0ac5 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -1340,6 +1340,9 @@
 #endif  // CONFIG_FILTER_INTRA
   } else {
     int16_t mode_ctx;
+
+    av1_collect_neighbors_ref_counts(xd);
+
     write_ref_frames(cm, xd, w);
 
     if (is_compound)