swscale/ppc: VSX-optimize yuv2rgb_full

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

This uses 32-bit mul, so POWER8 only.

The following output formats get about 4.5x speedup:

rgb24
  39980 UNITS in yuv2packed1,   32768 runs,      0 skips
   8774 UNITS in yuv2packed1,   32768 runs,      0 skips
bgr24
  40069 UNITS in yuv2packed1,   32768 runs,      0 skips
   8772 UNITS in yuv2packed1,   32766 runs,      2 skips
rgba
  39759 UNITS in yuv2packed1,   32768 runs,      0 skips
   8681 UNITS in yuv2packed1,   32767 runs,      1 skips
bgra
  39729 UNITS in yuv2packed1,   32768 runs,      0 skips
   8696 UNITS in yuv2packed1,   32766 runs,      2 skips
argb
  39766 UNITS in yuv2packed1,   32768 runs,      0 skips
   8672 UNITS in yuv2packed1,   32766 runs,      2 skips
bgra
  39784 UNITS in yuv2packed1,   32768 runs,      0 skips
   8659 UNITS in yuv2packed1,   32767 runs,      1 skips