libdpms: Guarantee validity of isSupported() right after construction
This reverts commit c37450e1, which fixed turning off the screen on Esc when locked, but broke some DPMS-related functions in PowerDevil, e.g. visibility of display turn-off timeouts in the "Energy Saving" KCM.
As a slight change from before though, this does not reintroduce the incorrect API docs statement that isSupported() would block while initializing. Callers can avoid the use of supportedChanged with a good conscience now, unless they track its state over time.
(cherry picked from commit 504f28d0)