drm/rockchip: fix race with kms hotplug and fbdev

drm/rockchip: fix race with kms hotplug and fbdev

Kms hotplug event may race into fbdev helper initial, 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

Change-Id: I8d594183cf8187131418a0096dde840cbf01ed6b
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>