video-converter: Fix for broken gamma remap with high bitdepth YUV output

Scale down the matrix before calculating RGB -> YUV matrix
otherwise offset values will be wrong

Fixing pipeline
videotestsrc ! video/x-raw,format=ARGB ! videoconvert gamma-mode=remap ! \
  video/x-raw,format=P010_10LE,colorimetry="bt2020"
24 jobs for !1432 with videoconverter-gamma in 37 seconds (queued for 5 seconds)
merge request