avcodec/vp9: add 64-bit ipred_dr_32x32_16 avx2 implementation

vp9_diag_downright_32x32_12bpp_c: 429.7
vp9_diag_downright_32x32_12bpp_sse2: 158.9
vp9_diag_downright_32x32_12bpp_ssse3: 144.6
vp9_diag_downright_32x32_12bpp_avx: 141.0
vp9_diag_downright_32x32_12bpp_avx2: 73.8

Almost 50% faster than avx implementation

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>