drm/rockchip: handle non-platform devices in rockchip_drm_endpoint_is_subdriver

As discussed at [1] rockchip_drm_endpoint_is_subdriver will currently always
return -ENODEV for non-platform-devices (e.g. external i2c bridges), what
makes them never being considered in rockchip_rgb_init.

As suggested at [1] this additionally adds a of_device_is_available for
the node found, which will work for both platform and non-platform devices.
Also we can return early for non-platform-devices if they are enabled,
as rockchip_sub_drivers contains exclusively platform-devices.

[1] https://lore.kernel.org/all/20210316182753.GA25685@earth.li/



Suggested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Alex Bee <knaerzche@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210914150756.85190-1-knaerzche@gmail.com