x86: hpeldsp: kill hpeldsp_mmx.c

before:
1987 decicycles in 8_x2, 262121 runs, 23 skips

after:
1902 decicycles in 8_x2, 262112 runs, 32 skips

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
x86: hpeldsp: mark _xy2 versions as approximate

Currently, only the mmx version is bitexact, the others (mmxext and
3dnow) are not, in spite of their naming.

Therefore, make their name more obvious. Also restore a comment that
was removed in 71155d7b.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
x86: hpeldsp: avg_pixels_xy2 for mmx2&3dnow

This is a port of the inline assembly of the mmx version to use the
pavg(us|)b instruction.

        8    16
mmx   1498  4355
mmx2  1242  3509

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