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

vp8/encoder/denoising.c

diff --git a/vp8/encoder/denoising.c b/vp8/encoder/denoising.c
index 0f0a36a..1a401a4 100644
--- a/vp8/encoder/denoising.c
+++ b/vp8/encoder/denoising.c
@@ -411,8 +411,8 @@
     MV_REFERENCE_FRAME zero_frame = x->best_zeromv_reference_frame;
 
     enum vp8_denoiser_decision decision = FILTER_BLOCK;
-    enum vp8_denoiser_decision decision_u = FILTER_BLOCK;
-    enum vp8_denoiser_decision decision_v = FILTER_BLOCK;
+    enum vp8_denoiser_decision decision_u = COPY_BLOCK;
+    enum vp8_denoiser_decision decision_v = COPY_BLOCK;
 
     if (zero_frame)
     {