Move the condition for effectively disabling the denoising
for speed 5 into the vp9_denoiser_denoise().

This is cleaner, and also moving the condition into vp9_denoiser_denoise
will keep the denoiser buffer updated with the current source.
This allows for more consistent behavior if speed is changed midstream.

Change-Id: Ia001f591c56e454bf724c3ae73c024badb183ef8