[NORMATIVE]Turn off palette mode for 4X4, 4X8 and 8X4 blocks

Compression loss on screen_content testset keyframes: 0.3%

BUG=aomedia:1367

Change-Id: Ifb5dd0d5b6362ec3cb29f43bdb6a8af6f38f6fe5
diff --git a/av1/common/blockd.h b/av1/common/blockd.h
index 1044f3a..2fe86cc 100644
--- a/av1/common/blockd.h
+++ b/av1/common/blockd.h
@@ -1180,7 +1180,7 @@
 static INLINE int av1_allow_palette(int allow_screen_content_tools,
                                     BLOCK_SIZE sb_type) {
   return allow_screen_content_tools && block_size_wide[sb_type] <= 64 &&
-         block_size_high[sb_type] <= 64;
+         block_size_high[sb_type] <= 64 && sb_type >= BLOCK_8X8;
 }
 
 // Returns sub-sampled dimensions of the given block.