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/encoder/block.h b/av1/encoder/block.h
index 2dd65d8..065e1a0 100644
--- a/av1/encoder/block.h
+++ b/av1/encoder/block.h
@@ -110,8 +110,8 @@
 } MvLimits;
 
 typedef struct {
-  uint8_t best_palette_color_map[MAX_SB_SQUARE];
-  float kmeans_data_buf[2 * MAX_SB_SQUARE];
+  uint8_t best_palette_color_map[MAX_PALETTE_SQUARE];
+  float kmeans_data_buf[2 * MAX_PALETTE_SQUARE];
 } PALETTE_BUFFER;
 
 typedef struct {