avcodec/x86/hpeldsp: fix crash on AMD K6-3+

There are instructions pavgb and pavgusb. Both instructions do the same
operation but they have different enconding. Pavgb exists in SSE (or
MMXEXT) instruction set and pavgusb exists in 3D-NOW instruction set.

livavcodec uses the macro PAVGB to select the proper instruction. However,
the function avg_pixels8_xy2 doesn't use this macro, it uses pavgb
directly.

As a consequence, the function avg_pixels8_xy2 crashes on AMD K6-2 and
K6-3 processors, because they have pavgusb, but not pavgb.

This bug seems to be introduced by commit
71155d7b4157fee44c0d3d0fc1b660ebfb9ccf46, "dsputil: x86: Convert mpeg4
qpel and dsputil avg to yasm"

Signed-off-by: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>