Adding vpx_sse_to_psnr() function.

Removing all copies of identical vp8_mse2psnr/vp9_mse2psnr functions.
Using vpx_sse_to_psnr() instead in all places.

Change-Id: I15beef9834d43d8fc8a8a7a2d1fc5de3d658fed8
13 files changed