x86: Move duplicated put_pixels{8|16}_mmx functions into their own file
x86: dsputil: Move avg_pixels8_mmx() out of rnd_template.c

The function is only instantiated once, so there is no point
in keeping it in a template file.
x86: dsputil: Move avg_pixels16_mmx() out of rnd_template.c

The function does not do any rounding, so there is no point in
keeping it in a round template file.
x86: dsputil: Move cavs and vc1-specific functions where they belong