libdpms: Guarantee validity of isSupported() right after construction

CCBUG: 338796

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)