Use single context model for mv diff

Reduce the context model size from 3 sets to 1 set for motion
vector difference coding in opt-ref-mv.

Change-Id: Iece1e01bb44f6a445c13787aa948400b8feef118

av1/common/mvref_common.h

@@ -270,6 +270,10 @@ static INLINE uint8_t av1_get_pred_diff_ctx(const int_mv pred_mv,
 static INLINE int av1_nmv_ctx(const uint8_t ref_mv_count,
                               const CANDIDATE_MV *ref_mv_stack, int ref,
                               int ref_mv_idx) {
+#if CONFIG_OPT_REF_MV
+  return 0;
+#endif
+
   if (ref_mv_idx < ref_mv_count &&
       ref_mv_stack[ref_mv_idx].weight >= REF_CAT_LEVEL)
     return ref_mv_stack[ref_mv_idx].pred_diff[ref];