libvpx

Small improvement in sub-pixel motion search

If the current best mv(namely, the search center) is still the best mv
after the first level search, the second level checks is skipped. This
patch doesn't change the bitstream. At speed 0, it speeds up the encoder
by 1% - 2%.

Change-Id: I054c91b884d3f7aef157436c061744562bd6506d