drm/fb-helper: Fix drm_fb_helper_hotplug_event() NULL ptr argument

drm_fb_helper_hotplug_event() should tolerate the fb_helper argument being
NULL. Commit 03a9606e ("drm/fb-helper: Avoid race with DRM userspace")
introduced a fb_helper dereference before the NULL check.
Fixup by moving the dereference after the NULL check.

Fixes: 03a9606e ("drm/fb-helper: Avoid race with DRM userspace")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190515132925.48867-1-noralf@tronnes.org

drivers/gpu/drm/drm_fb_helper.c

@@ -3031,7 +3031,6 @@ EXPORT_SYMBOL(drm_fb_helper_initial_config);
  */
 int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
 {
-	struct drm_device *dev = fb_helper->dev;
 	int err = 0;
 
 	if (!drm_fbdev_emulation || !fb_helper)
@@ -3044,13 +3043,13 @@ int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
 		return err;
 	}
 
-	if (!fb_helper->fb || !drm_master_internal_acquire(dev)) {
+	if (!fb_helper->fb || !drm_master_internal_acquire(fb_helper->dev)) {
 		fb_helper->delayed_hotplug = true;
 		mutex_unlock(&fb_helper->lock);
 		return err;
 	}
 
-	drm_master_internal_release(dev);
+	drm_master_internal_release(fb_helper->dev);
 
 	DRM_DEBUG_KMS("\n");