Emit brightnessSupportQueried with the proper value

Just because the sysfs helper succeeded, doesn't mean we actually have screen brightness support.
Only if maximum brightness greater than 0 is reported, we'll announce its support.
This fixes the brightness slider showing up in the KCM on desktop computers.

The battery monitor already has the same logic and correctly did not show brightness
sliders in this case.

Differential Revision: https://phabricator.kde.org/D2775