swscale/ppc: VSX-optimize non-full-chroma yuv2rgb_2

./ffmpeg -f lavfi -i yuvtestsrc=duration=1:size=1200x1440 -sws_flags fast_bilinear \
        -s 1200x720 -f null -vframes 100 -pix_fmt $i -nostats \
        -cpuflags 0 -v error -

32-bit mul, power8 only.

~2x speedup:

rgb24
  24431 UNITS in yuv2packed2,   16384 runs,      0 skips
  13783 UNITS in yuv2packed2,   16383 runs,      1 skips
bgr24
  24396 UNITS in yuv2packed2,   16384 runs,      0 skips
  14059 UNITS in yuv2packed2,   16384 runs,      0 skips
rgba
  26815 UNITS in yuv2packed2,   16383 runs,      1 skips
  12797 UNITS in yuv2packed2,   16383 runs,      1 skips
bgra
  27060 UNITS in yuv2packed2,   16384 runs,      0 skips
  13138 UNITS in yuv2packed2,   16384 runs,      0 skips
argb
  26998 UNITS in yuv2packed2,   16384 runs,      0 skips
  12728 UNITS in yuv2packed2,   16381 runs,      3 skips
bgra
  26651 UNITS in yuv2packed2,   16384 runs,      0 skips
  13124 UNITS in yuv2packed2,   16384 runs,      0 skips

This is a low speedup, but the x86 mmx version also gets only ~2x. The mmx version
is also heavily inaccurate, while the vsx version has high accuracy.