drm/fb-helper: Avoid NULL ptr dereference in fb_set_suspend()

drm_fb_helper_resume_worker() uses fb_helper->fbdev to call
fb_set_suspend() which dereferences the pointer.
Move sync-canceling of the resume worker in drm_fb_helper_fini() before
setting fb_helper->fbdev to NULL. Move dirty_work as well.
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1503940668-25883-2-git-send-email-noralf@tronnes.org

drivers/gpu/drm/drm_fb_helper.c

@@ -910,6 +910,9 @@ void drm_fb_helper_fini(struct drm_fb_helper *fb_helper)
 	if (!drm_fbdev_emulation || !fb_helper)
 		return;
 
+	cancel_work_sync(&fb_helper->resume_work);
+	cancel_work_sync(&fb_helper->dirty_work);
+
 	info = fb_helper->fbdev;
 	if (info) {
 		if (info->cmap.len)
@@ -918,9 +921,6 @@ void drm_fb_helper_fini(struct drm_fb_helper *fb_helper)
 	}
 	fb_helper->fbdev = NULL;
 
-	cancel_work_sync(&fb_helper->resume_work);
-	cancel_work_sync(&fb_helper->dirty_work);
-
 	mutex_lock(&kernel_fb_helper_lock);
 	if (!list_empty(&fb_helper->kernel_fb_list)) {
 		list_del(&fb_helper->kernel_fb_list);