vp9: Simplify the logic in denoiser SSE2 code.

Block size passed into denoiser filter is always >= BLOCK_8X8 (in
vp9_pick_inter_mode), it is not necessary to check smaller block
size. Passed the bitexact test on clips with different resolutions and
noise levels.

Change-Id: I19fa3195d18c27d9e5de60dc11cff1522ef3714e