drm/rockchip: analogix_dp: Use mutex rather than spinlock

On the Samsung Chromebook Plus I get this error with 4.14-rc3:

BUG: scheduling while atomic: kworker/3:1/50/0x00000002
Modules linked in:
CPU: 3 PID: 50 Comm: kworker/3:1 Not tainted 4.14.0-0.rc3-kevin #2
Hardware name: Google Kevin (DT)
Workqueue: events analogix_dp_psr_work
Call trace:
[<ffffff80080873b0>] dump_backtrace+0x0/0x320
[<ffffff80080876e4>] show_stack+0x14/0x20
[<ffffff8008606d38>] dump_stack+0x9c/0xbc
[<ffffff80080c6b5c>] __schedule_bug+0x4c/0x70
[<ffffff80086188c0>] __schedule+0x3f0/0x458
[<ffffff8008618960>] schedule+0x38/0xa0
[<ffffff800861c20c>] schedule_hrtimeout_range_clock+0x84/0xe8
[<ffffff800861c2a0>] schedule_hrtimeout_range+0x10/0x18
[<ffffff800861bcec>] usleep_range+0x64/0x78
[<ffffff8008415a6c>] analogix_dp_transfer+0x16c/0x340
[<ffffff8008412550>] analogix_dpaux_transfer+0x10/0x18
[<ffffff80083ceb14>] drm_dp_dpcd_access+0x4c/0xf0
[<ffffff80083cf614>] drm_dp_dpcd_write+0x1c/0x28
[<ffffff8008413b98>] analogix_dp_disable_psr+0x60/0xa8
[<ffffff800840da3c>] analogix_dp_psr_work+0x4c/0x90
[<ffffff80080bb09c>] process_one_work+0x1d4/0x348
[<ffffff80080bb258>] worker_thread+0x48/0x478
[<ffffff80080c11fc>] kthread+0x12c/0x130
[<ffffff8008084290>] ret_from_fork+0x10/0x18

Changing rockchip_dp_device::psr_lock to a mutex rather
than spinlock seems to fix the issue.

Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171004175346.11956-1-kernel@esmil.dk