x86: vc1dsp: Move ff_avg_vc1_mspel_mc00_mmxext out of dsputil_mmx.c
x86: dsputil: Drop aliasing of ff_put_pixels8_mmx to ff_put_pixels8_mmxext

The external assembly function uses mmxext instructions and should not be
masqueraded as an mmx-only function. Instead, use the mmx-only inline
assembly function.
x86: dsputil: Drop some unused function #defines
swscale: Add support for unscaled 8-bit Packed RGB -> Planar RGB

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>