(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