swscale: fix yuv2yuvX_8 assembly on x86

use_mmx_vfilter check/fix by commiter
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit b546023b9319cbaefb638a2eeac56bdbf53d6f8b)

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