video: rockchip: fix HDMI 4K resolution

3480 is not valid XRES, use 3840 as default.

Fixes: 05c65a82 ("video: rockchip: Support 4K resolution for rk3399, HDMI")
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com> # roc-rk3399-pc