A small bug fix in ext-refs on the RD mode selection

Change-Id: I25f14fec8e806cdf98d904488aaf200169def34d
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index f129c49..33febb5 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -10277,14 +10277,13 @@
       this_rd_thresh = (ref_frame == LAST3_FRAME)
                            ? rd_opt->threshes[segment_id][bsize][THR_LAST3]
                            : this_rd_thresh;
+      this_rd_thresh = (ref_frame == BWDREF_FRAME)
+                           ? rd_opt->threshes[segment_id][bsize][THR_BWDR]
+                           : this_rd_thresh;
 #endif  // CONFIG_EXT_REFS
       this_rd_thresh = (ref_frame == GOLDEN_FRAME)
                            ? rd_opt->threshes[segment_id][bsize][THR_GOLD]
                            : this_rd_thresh;
-#if CONFIG_EXT_REFS
-// TODO(zoeliu): To explore whether this_rd_thresh should consider
-//               BWDREF_FRAME and ALTREF_FRAME
-#endif  // CONFIG_EXT_REFS
 
       // TODO(any): Add search of the tx_type to improve rd performance at the
       // expense of speed.