drm/rockchip: cdn-dp: add cdn DP support for rk3399

Add support for cdn DP controller which is embedded in the rk3399
SoCs. The DP is compliant with DisplayPort Specification,
Version 1.3, This IP is compatible with the rockchip type-c PHY IP.
There is a uCPU in DP controller, it need a firmware to work,
please put the firmware file to /lib/firmware/rockchip/dptx.bin. The
uCPU in charge of aux communication and link training, the host use
mailbox to communicate with the ucpu.
The dclk pin_pol of vop must not be invert for DP.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
[seanpaul fixed up some races between the worker and modeset]
[seanpaul squashed ~15 commits from chromium.org gerrit]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
[groeck fixed compilation errors when building as module]
Signed-off-by: Guenter Roeck <groeck@chromium.org>