x86: hevc_mc: add AVX2 optimizations

x86: hevc_mc: add AVX2 optimizations

before
33304 decicycles in luma_bi_1, 523066 runs, 1222 skips
38138 decicycles in luma_bi_2, 523427 runs, 861 skips
13490 decicycles in luma_uni, 516138 runs, 8150 skips
after
20185 decicycles in luma_bi_1, 519970 runs, 4318 skips
24620 decicycles in luma_bi_2, 521024 runs, 3264 skips
10397 decicycles in luma_uni, 515715 runs, 8573 skips

Conflicts:
	libavcodec/x86/hevc_mc.asm
	libavcodec/x86/hevcdsp_init.c

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>