x86: More specific ifdefs for dsputil/hpeldsp init functions
x86: hpeldsp: Only compile MMX hpeldsp code if MMX is enabled
x86: hpeldsp: Move avg_pixels8_x2_mmx() out of hpeldsp_rnd_template.c

The function is only instantiated once, so there is no point
in keeping it in a template file.