vpx: [x86] add vp9_block_error_fp_avx2()

SSE2 asm vs AVX2 intrinsics speed gains:
blocksize   16: ~1.00
blocksize   64: ~1.17
blocksize  256: ~1.67
blocksize 1024: ~1.81

Change-Id: I2a86db239cf57e3ff617890ccb2d236aba83ad5e
3 files changed