libvpx

vp8 denoiser: fix to zero_mv mode selection.

In the current logic, if the sse for zero motion is smaller
than the sse for new_mv (i.e., best_sse), we may still end up
using the non-zero mv for denoising (if the magnitude of new_mv is above threshold).
This can happen for very noisy content, and can lead to artifacts.

This change ensures that we always use zero_mv (over new_mv) for
denoisng if sse_zero_mv <= best_sse.

Change-Id: I8ef9294d837b077013b77a46c9a71d17c648b48a