backends/drm: Undo fade-out effect upon unsuccessful DPMS Off

DrmOutput::setDrmDpmsMode() already takes care of reverting any
pending output pipeline changes, but the aboutToTurnOff signal from
setDpmsMode() needs an explicit wakeUp signal to cancel it out.

BUG: 477916
CCBUG: 481520


(cherry picked from commit 9e3e5675)