swscale/output: VSX-optimize nbps yuv2plane1

swscale/output: VSX-optimize nbps yuv2plane1

./ffmpeg_g -f rawvideo -pix_fmt rgb24 -s hd1080 -i /dev/zero -pix_fmt yuv420p9le \
-f null -vframes 100 -v error -nostats -

Speedups:
yuv2plane1_9BE_vsx	11.2042
yuv2plane1_9LE_vsx	11.156
yuv2plane1_10BE_vsx	9.89428
yuv2plane1_10LE_vsx	10.3637
yuv2plane1_12BE_vsx	9.71923
yuv2plane1_12LE_vsx	11.0404
yuv2plane1_14BE_vsx	10.1763
yuv2plane1_14LE_vsx	11.2728

Fate passes, each format tested with an image to video conversion.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>