vp8: Set default denoiser_decision to copy for UV channel.

Since the UV decision to denoise is based on Y, we need to set
the default/initial denoiser decision_u/v to COPY_BLOCK,
to make sure if no uv_denoiser is applied we still update
(uv)running_avg with source.

Change-Id: I5af1c2afbd40c498cd3de208bea88c837099b24d