Instead of using model_rd_for_sb() to estimate the cost and make the
decision on bmc/obmc, we use super_block_yrd/uvrd() to calculate and
compare the real rd costs of bmc and obmc.

Average bit-rate reduction(%) of obmc experiment:
derflr/derfhd/hevcmr/hevchd
2.353/TBD/TBD/TBD
Before the optimization, the coding gain was:
1.582/1.109/1.600/1.164

Note: there is still some mysterious bug because that compared to
the previous version, the performance at low bit rate drops a lot.

Change-Id: I8dbee04a272190f10516a3953c1ae690f8136766