libvpx

Turn on adaptive pred filter scheme for sub8x8 below 720p

For sequences of resolution below 720p, the encoder will check
intra prediction modes and inter prediction modes from LAST_FRAME.
This commit turns on adaptive prediction filter scheme for sub8x8
blocks, where inter prediction modes are enabled. For the test
sequence bus at CIF, the speed 2 runtime goes down from 17879 ms
to 16783 ms, i.e., 6% speed up. The compression performance of
derf set is down by -0.128%.

Change-Id: I01d5321a5ceab4e0666ac5be56c52d896c7a8d45