palette: Use some enums / defines

Change-Id: If5d4f6d261276d9cfdd59fd7779467d92f56c395
diff --git a/av1/encoder/rd.c b/av1/encoder/rd.c
index c4568a4..76d18ea 100644
--- a/av1/encoder/rd.c
+++ b/av1/encoder/rd.c
@@ -101,7 +101,7 @@
                     av1_default_palette_uv_size_prob[i], av1_palette_size_tree);
   }
 
-  for (i = 0; i < PALETTE_MAX_SIZE - 1; ++i) {
+  for (i = 0; i < PALETTE_SIZES; ++i) {
     for (j = 0; j < PALETTE_COLOR_INDEX_CONTEXTS; ++j) {
       av1_cost_tokens(cpi->palette_y_color_cost[i][j],
                       av1_default_palette_y_color_index_prob[i][j],