drm: vkms: check status of alloc_ordered_workqueue

alloc_ordered_workqueue may fail and return NULL.
The fix returns ENOMEM when it fails to avoid potential NULL
pointer dereference.
Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190309043628.6078-1-kjlu@umn.edu

drivers/gpu/drm/vkms/vkms_crtc.c

@@ -219,6 +219,8 @@ int vkms_crtc_init(struct drm_device *dev, struct drm_crtc *crtc,
 	spin_lock_init(&vkms_out->state_lock);
 
 	vkms_out->crc_workq = alloc_ordered_workqueue("vkms_crc_workq", 0);
+	if (!vkms_out->crc_workq)
+		return -ENOMEM;
 
 	return ret;
 }