backends/drm: hide cursor again

Before 6a99bfd2, the DrmBackend took care of checking this. Now it
needs to be explicitly checked in DrmOutput

src/backends/drm/drm_output.cpp

@@ -101,7 +101,7 @@ void DrmOutput::updateCursor()
         return;
     }
     const QImage cursorImage = cursor->image();
-    if (cursorImage.isNull()) {
+    if (cursorImage.isNull() || Cursors::self()->isCursorHidden()) {
         m_pipeline->setCursor(nullptr);
         return;
     }