monitor-manager-kms: Don't add GPU if it fails

This is a small mistake spotted while working on a solution
for #77. When a GPU fails to initialize, we're adding them
anyway, which might have pretty bad consequences when trying
to use these NULL GPUs.

Issue: #77

src/backends/native/meta-monitor-manager-kms.c

@@ -683,6 +683,7 @@ meta_monitor_manager_kms_initable_init (GInitable    *initable,
         {
           g_warning ("Failed to open secondary gpu '%s': %s",
                      gpu_path, secondary_error->message);
+          continue;
         }
 
       meta_monitor_manager_add_gpu (META_MONITOR_MANAGER (manager_kms),