move some SSIM-accumulation function for dsp/

This is in preparation for some SSE2 code.

And generally speaking, the whole SSIM code needs some
revamp: we're not averaging the SSIM value at each pixels
but just computing the overall SSIM value once, for the whole
plane. The former might be better than the latter.

Change-Id: I935784a917f84a18ef08dc5ec9a7b528abea46a5
5 files changed