drm/rockchip: fix race with kms hotplug and fbdev

According to the kerneldoc[0], should do fbdev setup before calling
drm_kms_helper_poll_init(), otherwise, Kms hotplug event may race
into fbdev helper initial, and fb_helper->dev may be NULL pointer,
that would cause the bug:
[    0.735411] [00000200] *pgd=00000000f6ffe003, *pud=00000000f6ffe003, *pmd=0000000000000000
[    0.736156] Internal error: Oops: 96000005 [#1] PREEMPT SMP
[    0.736648] Modules linked in:
[    0.736930] CPU: 2 PID: 20 Comm: kworker/2:0 Not tainted 4.4.41 #20
[    0.737480] Hardware name: Rockchip RK3399 Board rev2 (BOX) (DT)
[    0.738020] Workqueue: events cdn_dp_pd_event_work
[    0.738447] task: ffffffc0f21f3100 ti: ffffffc0f2218000 task.ti: ffffffc0f2218000
[    0.739109] PC is at mutex_lock+0x14/0x44
[    0.739469] LR is at drm_fb_helper_hotplug_event+0x30/0x114
[    0.756253] [<ffffff8008a344f4>] mutex_lock+0x14/0x44
[    0.756260] [<ffffff8008445708>] drm_fb_helper_hotplug_event+0x30/0x114
[    0.756271] [<ffffff8008473c84>] rockchip_drm_output_poll_changed+0x18/0x20
[    0.756280] [<ffffff8008439fcc>] drm_kms_helper_hotplug_event+0x28/0x34
[    0.756286] [<ffffff800846c444>] cdn_dp_pd_event_work+0x394/0x3c4
[    0.756295] [<ffffff80080b2b38>] process_one_work+0x218/0x3e0
[    0.756302] [<ffffff80080b3538>] worker_thread+0x2e8/0x404
[    0.756308] [<ffffff80080b7e70>] kthread+0xe8/0xf0
[    0.756316] [<ffffff8008082690>] ret_from_fork+0x10/0x40

[0]: https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms-helpers.html

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Reviewed-by: Sandy huang <sandy.huang@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1501575103-20136-1-git-send-email-mark.yao@rock-chips.com

drivers/gpu/drm/rockchip/rockchip_drm_drv.c

@@ -161,22 +161,21 @@ static int rockchip_drm_bind(struct device *dev)
 	 */
 	drm_dev->irq_enabled = true;
 
-	/* init kms poll for handling hpd */
-	drm_kms_helper_poll_init(drm_dev);
-
 	ret = rockchip_drm_fbdev_init(drm_dev);
 	if (ret)
-		goto err_kms_helper_poll_fini;
+		goto err_unbind_all;
+
+	/* init kms poll for handling hpd */
+	drm_kms_helper_poll_init(drm_dev);
 
 	ret = drm_dev_register(drm_dev, 0);
 	if (ret)
-		goto err_fbdev_fini;
+		goto err_kms_helper_poll_fini;
 
 	return 0;
-err_fbdev_fini:
-	rockchip_drm_fbdev_fini(drm_dev);
 err_kms_helper_poll_fini:
 	drm_kms_helper_poll_fini(drm_dev);
+	rockchip_drm_fbdev_fini(drm_dev);
 err_unbind_all:
 	component_unbind_all(dev, drm_dev);
 err_mode_config_cleanup: