Add AVX2 version of generate_grain_uv (4:2:0)
Add AVX2 version of generate_grain_uv (4:2:0)

gen_grain_uv_ar0_8bpc_420_c: 30131.8
gen_grain_uv_ar0_8bpc_420_avx2: 6600.4
gen_grain_uv_ar1_8bpc_420_c: 46110.5
gen_grain_uv_ar1_8bpc_420_avx2: 17887.2
gen_grain_uv_ar2_8bpc_420_c: 73593.2
gen_grain_uv_ar2_8bpc_420_avx2: 26918.6
gen_grain_uv_ar3_8bpc_420_c: 114499.3
gen_grain_uv_ar3_8bpc_420_avx2: 29804.6

xiph/rav1e,xiph/rav1e