Avoid using multiplication in context fetch

Replace the multiplication with shifts.

Change-Id: I245efaddea2019d789179569e82e81bb7cb97715
diff --git a/av1/common/enums.h b/av1/common/enums.h
index 3d1b70f..7fa17e5 100644
--- a/av1/common/enums.h
+++ b/av1/common/enums.h
@@ -251,6 +251,7 @@
 #define MAX_TX_SQUARE (MAX_TX_SIZE * MAX_TX_SIZE)
 
 // Pad 4 extra columns to remove horizontal availability check.
+#define TX_PAD_HOR_LOG2 2
 #define TX_PAD_HOR 4
 // Pad 6 extra rows (2 on top and 4 on bottom) to remove vertical availability
 // check.