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

src/backends/drm/drm_output.cpp

@@ -106,7 +106,10 @@ DrmOutput::DrmOutput(const std::shared_ptr<DrmConnector> &conn)
     m_turnOffTimer.setSingleShot(true);
     m_turnOffTimer.setInterval(dimAnimationTime());
     connect(&m_turnOffTimer, &QTimer::timeout, this, [this] {
-        setDrmDpmsMode(DpmsMode::Off);
+        if (!setDrmDpmsMode(DpmsMode::Off)) {
+            // in case of failure, undo aboutToTurnOff() from setDpmsMode()
+            Q_EMIT wakeUp();
+        }
     });
 }