Properly reset the skip_mode element in mb_mode_info

The skip_mode element might re-use prior frame's coding decision
for a current coding block rate-distortion search. Properly reset
it to be zero for regular rate-distortion mode search.

This improves the coding performance for ext-skip by 0.07% for
lowres.

Change-Id: Idbda5b441e3eb844e03ca07bd174b4b7f8a7cb59

av1/encoder/encodeframe.c

diff --git a/av1/encoder/encodeframe.c b/av1/encoder/encodeframe.c
index f633a78..a8b1a5f 100644
--- a/av1/encoder/encodeframe.c
+++ b/av1/encoder/encodeframe.c
@@ -677,6 +677,11 @@
 
     // Set to zero to make sure we do not use the previous encoded frame stats
     mbmi->skip = 0;
+
+#if CONFIG_EXT_SKIP
+    // Reset skip mode flag.
+    mbmi->skip_mode = 0;
+#endif
   }
 
   x->skip_chroma_rd =