[platforms/drm] only enable output on key press not release

Otherwise if powerdevil for example is configured to turn screen off on
the power key press, we will turn it right back on when we get key
release event.

Suggested-by: David Edmundson <kde@davidedmundson.co.uk>

src/plugins/platforms/drm/drm_inputeventfilter.cpp

@@ -42,9 +42,11 @@ bool DpmsInputEventFilter::wheelEvent(QWheelEvent *event)
 
 bool DpmsInputEventFilter::keyEvent(QKeyEvent *event)
 {
-    Q_UNUSED(event)
-    notify();
-    return true;
+    if (event->type() == QKeyEvent::KeyPress) {
+        notify();
+        return true;
+    }
+    return false;
 }
 
 bool DpmsInputEventFilter::touchDown(qint32 id, const QPointF &pos, quint32 time)