backends/native: Move KMS fd management to monitor manager

Move finding, opening and managment of the KMS file descriptor to
MetaMonitorManagerKms. This means that the monitor manager creation can
now fail, both if more than one GPU with connectors is discovered, or
if finding or opening the primary GPU fails.

https://bugzilla.gnome.org/show_bug.cgi?id=785381

src/tests/meta-backend-test.c

@@ -36,7 +36,8 @@ meta_backend_test_init (MetaBackendTest *backend_test)
 }
 
 static MetaMonitorManager *
-meta_backend_test_create_monitor_manager (MetaBackend *backend)
+meta_backend_test_create_monitor_manager (MetaBackend *backend,
+                                          GError     **error)
 {
   return g_object_new (META_TYPE_MONITOR_MANAGER_TEST, NULL);
 }