x86/videodsp: add emulated_edge_mc_mmxext

This also changes hfix8_mmx and above to use mmx regs instead of
gprs, and makes emulated_edge_mc_sse and emulated_edge_mc_sse2 use
mmxext hfix and hvar functions instead of mmx where possible.

This is mostly in preparation for an ssse3 version.

Signed-off-by: James Almer <jamrial@gmail.com>

code is about 1 cpu cycle faster approximately

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