avcodec/dsputil/huffyuv: move diff int16 and add int16 to dsputil

This also fixes masking the bits

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/x86/dsp: add_int16_mmx / add_int16_sse2

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