drm/rockchip/dsi: check phy_cfg_clk only for RK3399

For RK3399, the phy_cfg_clk is a required clock, if phy_cfg_clk is
disabled, MIPI phy can not work. Let's return a error if there is no
phy_cfg_clk in dts property, when the pdata match RK3399.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1490147691-4489-2-git-send-email-zyw@rock-chips.com