Refine rd_pick_skip_mode

Set best_mode_skippable to 1 if skip_mode is the best mode.
This flag is used in partition search to determine enabling
ML based breakout.

Tested BasketballDrill_832x480_50 (20 frames, bitrate=800, Speed 1),
it is 3% faster.

STATS_CHANGED

Change-Id: Id53399fde486ca568f41d377c45be4d69f47a25d

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index a3fa354..b16773c 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -10463,7 +10463,7 @@
 
     search_state->best_rd = rd_cost->rdcost;
     search_state->best_skip2 = 1;
-    search_state->best_mode_skippable = (skip_mode_rd_stats.sse == 0);
+    search_state->best_mode_skippable = 1;
 
     x->skip = 1;
   }