Make get_entropy_context() support 2x2 transform block

Change-Id: Icbb4e686b4a5e7fdd1de76d412f43fdf1bf8830a

av1/common/entropy.h

diff --git a/av1/common/entropy.h b/av1/common/entropy.h
index f3b8bd1..43f849f 100644
--- a/av1/common/entropy.h
+++ b/av1/common/entropy.h
@@ -219,6 +219,12 @@
   ENTROPY_CONTEXT above_ec = 0, left_ec = 0;
 
   switch (tx_size) {
+#if CONFIG_CB4X4
+    case TX_2X2:
+      above_ec = a[0] != 0;
+      left_ec = l[0] != 0;
+      break;
+#endif
     case TX_4X4:
       above_ec = a[0] != 0;
       left_ec = l[0] != 0;