Do not allow palette mode for partition sizes larger than 64x64

Tested on screen_content, it has very little impact on compression quality.

Change-Id: I49c7d5e8de8420c0d2af1b327c084a2667cfea40
diff --git a/av1/common/blockd.h b/av1/common/blockd.h
index 5d12740..1644057 100644
--- a/av1/common/blockd.h
+++ b/av1/common/blockd.h
@@ -1389,7 +1389,7 @@
 static INLINE int av1_allow_palette(int allow_screen_content_tools,
                                     BLOCK_SIZE sb_type) {
   return allow_screen_content_tools && sb_type >= BLOCK_8X8 &&
-         sb_type <= BLOCK_LARGEST;
+         sb_type <= BLOCK_64X64;
 }
 
 // Returns sub-sampled dimensions of the given block.