platform/drm: add dpmsFinishOn to atomicEnable

Without it the output will stay blank after enabling

src/plugins/platforms/drm/drm_output.cpp

@@ -315,6 +315,7 @@ void DrmOutput::atomicEnable()
         m_atomicOffPending = false;
     }
     m_backend->enableOutput(this, true);
+    dpmsFinishOn();
 
     if (Compositor *compositor = Compositor::self()) {
         compositor->addRepaintFull();