avcodec/cbs_av1: Add tx mode enum values

Signed-off-by: Fei Wang <fei.w.wang@intel.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>

libavcodec/av1.h

@@ -175,6 +175,13 @@ enum {
     AV1_RESTORE_SWITCHABLE = 3,
 };
 
+// TX mode (section 6.8.21)
+enum {
+    AV1_ONLY_4X4        = 0,
+    AV1_TX_MODE_LARGEST = 1,
+    AV1_TX_MODE_SELECT  = 2,
+};
+
 // Sequence Headers are actually unbounded because one can use
 // an arbitrary number of leading zeroes when encoding via uvlc.
 // The following estimate is based around using the lowest number

libavcodec/cbs_av1_syntax_template.c

@@ -1028,9 +1028,9 @@ static int FUNC(read_tx_mode)(CodedBitstreamContext *ctx, RWContext *rw,
     int err;
 
     if (priv->coded_lossless)
-        infer(tx_mode, 0);
+        infer(tx_mode, AV1_ONLY_4X4);
     else
-        increment(tx_mode, 1, 2);
+        increment(tx_mode, AV1_TX_MODE_LARGEST, AV1_TX_MODE_SELECT);
 
     return 0;
 }