Fix a bug in calculating delta in VP9 denoiser.
Fix a bug in calculating delta in VP9 denoiser.

When calculating delta in VP8 denoiser, since the block size is fixed to 16x16,
the divisor is 256, which is the number of the pixel.
But in VP9, the block size varies, the divisor should correspond to the block
size.

Change-Id: Ibdc1e5d23ba8c788b0d0dc6d406bcdfc34c1b142

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