rockchip: video: edp: Add rk3399 support

rockchip: video: edp: Add rk3399 support

According to linux commit "drm/rockchip: analogix_dp: add rk3399 eDP
support" (82872e42bb1501dd9e60ca430f4bae45a469aa64), rk3288 and rk3399
eDP IPs are nearly the same, the difference is in the grf register
(SOC_CON6 versus SOC_CON20). So, change the code to use the right
register on each IP.

The clocks don't seem to be the same, the eDP clock is not at index 1
on rk3399, so don't try changing the clock at index 1 to rate 0 on
rk3399.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Tested-by: Peter Robinson <pbrobinson@gmail.com>