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

drivers/gpu/drm/rockchip/rockchip_drm_drv.c

@@ -272,10 +272,17 @@ int rockchip_drm_endpoint_is_subdriver(struct device_node *ep)
 		return -ENODEV;
 
 	/* status disabled will prevent creation of platform-devices */
+	if (!of_device_is_available(node)) {
+		of_node_put(node);
+		return -ENODEV;
+	}
+
 	pdev = of_find_device_by_node(node);
 	of_node_put(node);
+
+	/* enabled non-platform-devices can immediately return here */
 	if (!pdev)
-		return -ENODEV;
+		return false;
 
 	/*
 	 * All rockchip subdrivers have probed at this point, so