Refactor transform size coding

Introduce the transform block partition depth macro definition.

Change-Id: I218dc77a77c8e967da4d270d4ec0d7691b712a5f
diff --git a/av1/common/enums.h b/av1/common/enums.h
index 0a1f7a3..701d4b9 100644
--- a/av1/common/enums.h
+++ b/av1/common/enums.h
@@ -148,6 +148,8 @@
   TX_INVALID = 255          // Invalid transform size
 } TX_SIZE;
 
+#define MAX_TX_DEPTH (TX_32X32 - TX_4X4)
+
 #define MAX_TX_SIZE_LOG2 5
 #define MAX_TX_SIZE (1 << MAX_TX_SIZE_LOG2)
 #define MIN_TX_SIZE_LOG2 2