Search collocated reference block in 16x16 unit

Use 16x16 block resolution for collocated reference motion vector
search.

Change-Id: I1091b5b178e255eb6cc0b994de360994f7661b79
diff --git a/av1/common/mvref_common.c b/av1/common/mvref_common.c
index 0f3f949..2b63fd5 100644
--- a/av1/common/mvref_common.c
+++ b/av1/common/mvref_common.c
@@ -399,8 +399,8 @@
     int blk_row, blk_col;
     int coll_blk_count = 0;
 
-    for (blk_row = 0; blk_row < xd->n8_h; ++blk_row) {
-      for (blk_col = 0; blk_col < xd->n8_w; ++blk_col) {
+    for (blk_row = 0; blk_row < xd->n8_h; blk_row += 2) {
+      for (blk_col = 0; blk_col < xd->n8_w; blk_col += 2) {
         coll_blk_count += add_col_ref_mv(
             cm, prev_frame_mvs_base, xd, mi_row, mi_col, ref_frame, blk_row,
             blk_col, refmv_count, ref_mv_stack, mode_context);