Fix crash on hotplugging displays while switched to another TTY

Currently, we inhibit renderloops when switching to another TTY, and unhibit
when switching back. When we hotplug a display while switched to another TTY,
its renderloop isn't inhibited by default, yet when we switch back, we try
to uninhibit the fresh renderloop, which triggers an assert.

Inhibiting newly created outputs while switched to another TTY fixes this crash.

BUG: 435388
FIXED-IN: 5.21

src/plugins/platforms/drm/drm_backend.cpp

@@ -279,6 +279,10 @@ bool DrmBackend::initialize()
 
 void DrmBackend::addOutput(DrmOutput *o)
 {
+    if (!m_active) {
+        o->renderLoop()->inhibit();
+    }
+
     m_outputs.append(o);
     m_enabledOutputs.append(o);
     emit o->gpu()->outputEnabled(o);