Fix multi-thread encoding for cb4x4 mode

This commit makes the encoder to properly account for all transform
block sizes when combining statistics from encoding threads.

Change-Id: I010acd3b247dc890f63756d3d1436b1fb52ea2d9

av1/encoder/encoder.c

diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index b4cc578..f652fe0 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -4781,7 +4781,7 @@
 
   av1_update_reference_frames(cpi);
 
-  for (t = TX_4X4; t < TX_SIZES; t++)
+  for (t = 0; t < TX_SIZES; t++)
     av1_full_to_model_counts(cpi->td.counts->coef[t],
                              cpi->td.rd_counts.coef_counts[t]);