Display parameters exposed to userland as percentages (backlight, brightness,

Display parameters exposed to userland as percentages (backlight, brightness,
contrast) are not valid if they only have one state, i.e. minimum and maximum
values being equal.

Do not expose them to userland in this case, for wsconsctl would attempt to
divide by zero (which is known to have unwelcome consequences).

This allows display drivers trusting not-so-reliable sources (fdt, bogus
hardware...) to not have to perform those checks themselves.

Found the hard way by daniel@. No firm consensus on this workaround, using
one seniority point here, will revert if this spawns complaints.