Fix rate estimate issue in transform block partition coding

This commit fixes the over count issue in the recursive transform
block partition rate cost estimation. It improves the compression
performance by about 0.45%.

Change-Id: I01ccda954ed0e120263977472c1c759c3c67170c

vp9/encoder/vp9_rdopt.c

@@ -1233,7 +1233,8 @@ static void select_tx_block(const VP9_COMP *cpi, MACROBLOCK *x,
   if (cpi->common.tx_mode == TX_MODE_SELECT || tx_size == TX_4X4) {
     tx_block_rd_b(x, tx_size, blk_row, blk_col, plane, block,
                   plane_bsize, ta, tl, rate, dist, bsse, skip);
-    *rate += 256;
+    if (tx_size > TX_4X4)
+      *rate += 256;
     this_rd = RDCOST(x->rdmult, x->rddiv, *rate, *dist);
   }