Use tx_size_from_tx_mode() to set tx_size for skip blocks

Unify the use case of largest transform block size fetch for skip
blocks.

Change-Id: I5dd60eb74b266fa7cd7944d8f823dbdae75e90af
diff --git a/av1/encoder/encodeframe.c b/av1/encoder/encodeframe.c
index 73283f6..b08c370 100644
--- a/av1/encoder/encodeframe.c
+++ b/av1/encoder/encodeframe.c
@@ -5487,8 +5487,7 @@
       if (xd->lossless[mbmi->segment_id]) tx_size = TX_4X4;
     }
 #else
-      tx_size = AOMMIN(tx_mode_to_biggest_tx_size[cm->tx_mode],
-                       max_txsize_lookup[bsize]);
+      tx_size = tx_size_from_tx_mode(bsize, cm->tx_mode, is_inter);
 #endif
     else
       tx_size = (bsize >= BLOCK_8X8) ? tx_size : TX_4X4;