FROMLIST: drm/rockchip: fix fbdev crash when not use DRM_FBDEV_EMULATION

FROMLIST: drm/rockchip: fix fbdev crash when not use DRM_FBDEV_EMULATION

[    1.162571] Unable to handle kernel NULL pointer dereference at virtual address 00000200
[    1.165656] Modules linked in:
[    1.165941] CPU: 5 PID: 143 Comm: kworker/5:2 Not tainted 4.4.15 #237
[    1.166506] Hardware name: Rockchip RK3399 Evaluation Board v1 (Android) (DT)
[    1.167153] Workqueue: events output_poll_execute
[    1.168231] PC is at mutex_lock+0x14/0x44
[    1.168586] LR is at drm_fb_helper_hotplug_event+0x28/0xcc
[    1.172192] [<ffffff8008982110>] mutex_lock+0x14/0x44
[    1.172196] [<ffffff80084025a4>] drm_fb_helper_hotplug_event+0x28/0xcc
[    1.172201] [<ffffff8008427ae4>] rockchip_drm_output_poll_changed+0x14/0x1c
[    1.172204] [<ffffff80083f7c4c>] drm_kms_helper_hotplug_event+0x28/0x34
[    1.172207] [<ffffff80083f7ddc>] output_poll_execute+0x150/0x198
[    1.172212] [<ffffff80080b0ea8>] process_one_work+0x218/0x3dc
[    1.172215] [<ffffff80080b1578>] worker_thread+0x24c/0x374
[    1.172217] [<ffffff80080b5bcc>] kthread+0xdc/0xe4
[    1.172222] [<ffffff8008084cd0>] ret_from_fork+0x10/0x40

Change-Id: I681b9260ad7f2e3cae5cd08a109dad89b3575c2c
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
(am from https://patchwork.kernel.org/patch/9262523)