avcodec/x86/dsputil_init: remove duplicated sse2 idct init

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/x86/dsputil_init: move ff_idct_xvid_mmxext init

This decreases the diff to libav

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