Fix spacemacs|do-after-display-system-init

Checking for system-type is not enough accurate, it broke when using
an X server under OS X because system-type is darwin but the
initialized display is x-initialized not ns-initialized.

core/core-display-init.el

@@ -30,9 +30,9 @@ the display system initialized."
   "If the display-system is initialized, run `BODY', otherwise,
 add it to a queue of actions to perform after the first graphical frame is
 created."
-  `(let ((init (cond ((eq system-type 'darwin) 'ns-initialized)
-                     ((eq system-type 'windows-nt) 'w32-initialized)
-                     ((eq system-type 'gnu/linux) 'x-initialized)
+  `(let ((init (cond ((bound-and-true-p ns-initialized) 'ns-initialized)
+                     ((bound-and-true-p w32-initialized) 'w32-initialized)
+                     ((bound-and-true-p x-initialized) 'x-initialized)
                      (t 't))))           ; fallback to normal loading behavior
      (if (symbol-value init)
          (progn