Modify calling of NEON code in sub-pixel search

In vp8_find_best_sub_pixel_step_iteratively(), many times xoffset
and yoffset are specific values - (4,0) (0,4) and (4,4). Modified
code to call simplified NEON version at these specific offsets to
help with the performance.

Change-Id: Iaf896a0f7aae4697bd36a49e182525dd1ef1ab4d