Add support for the max bpc connector property.
Useful for lowering bandwidth requirements on systems that can't maintain a stable signal. Or for increasing the displayed colour depth on kernel drivers that default to too low (allegedly amdgpu has done this in the past).
Hopefully fixes bugs like: https://launchpad.net/bugs/1929209
We might also want to do similar in plymouth (https://gitlab.freedesktop.org/plymouth/plymouth/-/issues/102).