Reduce the ref mv search region for sub8x8 blocks

Reduce the reference motion vector search region over the spatial
neighbor blocks for sub8x8 block sizes, in order to reduce the
worst case context model parsing latency.

Change-Id: I77a2a25483836cc02cf1784c93566fa7cff40fc8

av1/common/mvref_common.c

@@ -730,12 +730,20 @@ static void setup_ref_mv_list(const AV1_COMMON *cm, const MACROBLOCKD *xd,
   // Find valid maximum row/col offset.
   if (xd->up_available) {
     max_row_offset = -(MVREF_ROWS << 1) + row_adj;
+#if CONFIG_OPT_REF_MV
+    if (xd->n8_h < mi_size_high[BLOCK_8X8])
+      max_row_offset = -(2 << 1) + row_adj;
+#endif
     max_row_offset =
         find_valid_row_offset(tile, mi_row, cm->mi_rows, cm, max_row_offset);
   }
 
   if (xd->left_available) {
     max_col_offset = -(MVREF_COLS << 1) + col_adj;
+#if CONFIG_OPT_REF_MV
+    if (xd->n8_w < mi_size_wide[BLOCK_8X8])
+      max_col_offset = -(2 << 1) + col_adj;
+#endif
     max_col_offset = find_valid_col_offset(tile, mi_col, max_col_offset);
   }
 
@@ -860,6 +868,7 @@ static void setup_ref_mv_list(const AV1_COMMON *cm, const MACROBLOCKD *xd,
                 gm_mv_candidates,
 #endif  // USE_CUR_GM_REFMV
                 refmv_count);
+
   for (int idx = 2; idx <= MVREF_ROWS; ++idx) {
     const int row_offset = -(idx << 1) + 1 + row_adj;
     const int col_offset = -(idx << 1) + 1 + col_adj;