Cleanup some fragile aspects of rd_pick_partition.
(1) Explicitly reset RD stats for each partition. Earlier, PARTITION_SPLIT was the only one resetting the RD_STATS in 'sum_rdc'. But this was working because: - PARTITION_SPLIT was tried before VERT, HORZ, VERT_4 and HORZ_4; and - RD cost calculations in VERT, HORZ, VERT_4 and HORZ_4 partitions implicitly discarded existing value in sum_rdc However, that was very fragile; explicitly resetting the stats every time is much safer. (2) Using a separate variable 'temp_best_rd_cost' was fragile as someone may forget to update the same. So, we use best_rdc.rdcost directly. BUG=aomedia:1246 Change-Id: Icd75f25c34bb0f1806e691784648bcffce2417e6