drm: Also release the pipeline buffer when DrmOutput::releaseBuffers()

src/plugins/platforms/drm/drm_output.cpp

@@ -76,6 +76,7 @@ void DrmOutput::teardown()
 
 void DrmOutput::releaseBuffers()
 {
+    m_pipeline->setPrimaryBuffer(nullptr);
     m_crtc->setCurrent(nullptr);
     m_crtc->setNext(nullptr);
     m_primaryPlane->setCurrent(nullptr);