Let vp9_nb_mvs_inconsistency call log2 just once

Let vp9_nb_mvs_inconsistency call log2 just once

The bahavior of this function is to compute log2 of mv difference,
i.e. min log2(1 + row_diff * row_diff + col_diff * col_diff)
against available neghbor mvs.
Since the log2 is monotonic increasing, we can compute
min row_diff * row_diff + col_diff * col_diff first
then apply log2 in the end

non_greedy_mv encoding time is reduced by 1.5%

Change-Id: I70d40060e2621daec27229f1f6d9fea0286aa04e