Refactor mc reference block fetch in denoiser

This commit refactors the motion compensated reference block fetch
process in denoiser. It skips the stage that generates motion
compensated reference block if denoiser decides to use copy block
mode. For high motion clips, this could speed up the denoising
process by about 10%.

Change-Id: I8ef4fa5fe766a8c4529119b9ec01faefb3d4ef53