When the coding block size has one side in length of 64 and above,
scale up the mfmv reference search step size from 8 to 16 along
that direction. The midres coding stats get 0.02% better. Among
all the finished hdres points, no negative results showed up.

BUG=aomedia:1379

Change-Id: I70ab7a9f9d1cf365d8ed1e06dbede307b6bc46ec