Change to rd costing for CONFIG_VAR_TX

In select_tx_block I believe the rd cost was ignoring the bits
needed to encode the split bit.

Change-Id: Iacbf705b880db9a68967a994406ba90ecf693ab8

vp10/encoder/rdopt.c

diff --git a/vp10/encoder/rdopt.c b/vp10/encoder/rdopt.c
index 6611868..bd7be7f 100644
--- a/vp10/encoder/rdopt.c
+++ b/vp10/encoder/rdopt.c
@@ -2364,7 +2364,7 @@
       sum_dist += this_dist;
       sum_bsse += this_bsse;
       all_skip &= this_skip;
-      tmp_rd += RDCOST(x->rdmult, x->rddiv, this_rate, this_dist);
+      tmp_rd = RDCOST(x->rdmult, x->rddiv, sum_rate, sum_dist);
       if (this_rd < tmp_rd)
         break;
     }