x86/hevc: add 12bits support for MC

cherry picked from commit 3fcb7a4595a6f40100a22110a5805e3b7510c0fd

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

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