Simplify computation in av1_get_tx_scale()

Change-Id: I36fd428a46c7b54a558d568c9a73c690fb4f6586

av1/common/idct.c

diff --git a/av1/common/idct.c b/av1/common/idct.c
index db7bc92..be1ef7e 100644
--- a/av1/common/idct.c
+++ b/av1/common/idct.c
@@ -22,7 +22,8 @@
 
 int av1_get_tx_scale(const TX_SIZE tx_size) {
   const int pels = tx_size_2d[tx_size];
-  return (pels > 256) + (pels > 1024) + (pels > 4096);
+  // Largest possible pels is 4096 (64x64).
+  return (pels > 256) + (pels > 1024);
 }
 
 // NOTE: The implementation of all inverses need to be aware of the fact