AV1 RT: Don't calculate sb_source metric for non-modeled cost

Switch off sb source metric for low res as modeled cost is not used
Bitexact on speed8 and very small speed increase.

Change-Id: I6a118e044129d8fd1535f4dbacf50fe8ef9b43b5

av1/encoder/speed_features.c

diff --git a/av1/encoder/speed_features.c b/av1/encoder/speed_features.c
index 5d1177c..4a69840 100644
--- a/av1/encoder/speed_features.c
+++ b/av1/encoder/speed_features.c
@@ -253,6 +253,7 @@
     if (speed >= 8) {
       sf->rt_sf.use_modeled_non_rd_cost = 0;
       sf->rt_sf.use_nonrd_filter_search = 0;
+      sf->rt_sf.source_metrics_sb_nonrd = 0;
     }
   }
   if (is_360p_or_larger) {