libvpx

Fix a bug in VP9 denoiser.

When the first try of denoising turns out to be too much,
we will use a softer filter by adopting an adjustment to
make the result closer to original pixel (as in VP8 denoiser).
The old code made the adjustment in the wrong direction.

Change-Id: I84e28fa9e01eef47c5a37d5a2e6d3d378a06786b