Directly split the block in partition search

Directly split the block in partition search

This patch allows the encoder to directly split the block
in partition search, therefore skip searching NONE. It
computes a score which measures whether 16x16 motion vectors
from the first pass in the current block are consistent with
each others. If they are inconsistent and we have enough Q
to encode, split the block directly, and skip searching NONE.

This feature is under flag CONFIG_FP_MB_STATS. In speed 2,
it further gives a speedup of 3-8% on sample yt clips as
compared to the previous version under the same flag. Overall,
the features under the flag will give 7-15% on typical yt
clips at up to 6000kbps data rate. The speedup at very high
data rate is not significant.

For hard stdhd clips:
park_joy_1080p @ 15000kbps:       504541ms -> 506293ms (-0.35%)
pedestrian_area_1080p @ 2000kbps: 326610ms -> 290090ms (+11.2%)

The compression performance using the features under the flag:
derf: -0.068%
yt:   -0.189%
hd:   -0.318%
stdhd:-0.183%

To use the feature, set CONFIG_FP_MB_STATS and turn on
cpi->use_fp_mb_stats.

Change-Id: Iad58a2966515c8861aa9eb211565b1864048d47f