Make av1_update_txb_coeff_cost() check condition support cb4x4

Replace hard coded numbers with macro defs.

Change-Id: I125ef4e4c8c3aead182c583522450626b730bbb3
diff --git a/av1/encoder/rdopt.h b/av1/encoder/rdopt.h
index 8d1df95..301e5aa 100644
--- a/av1/encoder/rdopt.h
+++ b/av1/encoder/rdopt.h
@@ -48,8 +48,8 @@
   }
 #endif
 
-  assert(blk_row < 16);
-  assert(blk_col < 16);
+  assert(blk_row < TXB_COEFF_COST_MAP_SIZE);
+  assert(blk_col < TXB_COEFF_COST_MAP_SIZE);
 }
 #endif