libvpx

Enable motion field based mode seach skip

This commit allows the encoder to check the above and left neighbor
blocks' reference frames and motion vectors. If they are all
consistent, skip checking the NEARMV and ZEROMV modes. This is
enabled in speed 3. The coding performance is improved:

pedestrian area 1080p at 2000 kbps,
from  74773 b/f, 41.101 dB, 198064 ms
to    74795 b/f, 41.099 dB, 193078 ms

park joy 1080p at 15000 kbps,
from 290727 b/f, 30.640 dB, 609113 ms
to   290558 b/f, 30.630 dB, 592815 ms

Overall compression performance of speed 3 is changed
derf  -0.171%
stdhd -0.168%

Change-Id: I8d47dd543a5f90d7a1c583f74035b926b6704b95