backends/drm: don't use ColorDescription::toOther for calculating channel factors
ColorDescription::toOther adjusts the brightness from the original color description to the new one, which makes it unsuitable for checking the brightness after the transformation. Instead, a new helper in Colorimetry is added that does only the desired calculation without any brightness changes.
(cherry picked from commit 22c519f0)
Co-authored-by: Xaver Hugl xaver.hugl@gmail.com