backends/drm: don't set the dpms mode to AboutToTurnOff if the screen is already off

Otherwise, DrmOutput::setDrmDpmsMode will inhibit the renderloop for a second time, and it'll
never be completely un-inhibited, which means the screen stays off.

src/backends/drm/drm_output.cpp

@@ -142,6 +142,9 @@ QList<std::shared_ptr<OutputMode>> DrmOutput::getModes() const
 
 void DrmOutput::setDpmsMode(DpmsMode mode)
 {
+    if (mode == dpmsMode()) {
+        return;
+    }
     if (mode == DpmsMode::Off) {
         if (!m_turnOffTimer.isActive()) {
             updateDpmsMode(DpmsMode::AboutToTurnOff);