Fix Qt6 build regressions introduced by 0cd453690b

Hard to avoid as long as we don't have CI coverage yet, but that will take
a bit more time, we need KWin (and all its dependencies) to fully build
first.

src/backends/drm/virtual_egl_gbm_layer.cpp

@@ -90,7 +90,7 @@ bool VirtualEglGbmLayer::endRendering(const QRegion &damagedRegion)
         m_currentBuffer = buffer;
         m_currentDamage = damagedRegion;
     }
-    return buffer;
+    return !buffer.isNull();
 }
 
 QRegion VirtualEglGbmLayer::currentDamage() const