Change plane_rd_mult inter uv param from 5 to 6

This will improve the lowres performance by 0.1%
No significant change on midres and hdres

Change-Id: I0c6e69bf48596e7e5ad49631ed9f9be73bbf3c9a

av1/encoder/encodemb.c

diff --git a/av1/encoder/encodemb.c b/av1/encoder/encodemb.c
index 347926f..e22b60c 100644
--- a/av1/encoder/encodemb.c
+++ b/av1/encoder/encodemb.c
@@ -115,7 +115,7 @@
 #if CONFIG_EC_ADAPT
   { 10, 7 }, { 8, 5 },
 #else
-  { 10, 6 }, { 8, 5 },
+  { 10, 6 }, { 8, 6 },
 #endif
 };