drm: rcar-du: Add support for the nomodeset kernel parameter

According to disable Documentation/admin-guide/kernel-parameters.txt, this
parameter can be used to disable kernel modesetting.

DRM drivers will not perform display-mode changes or accelerated rendering
and only the system framebuffer will be available if it was set-up.

But only a few DRM drivers currently check for nomodeset, make this driver
to also support the command line parameter.
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211217003752.3946210-32-javierm@redhat.com

drivers/gpu/drm/rcar-du/rcar_du_drv.c

@@ -701,6 +701,9 @@ static struct platform_driver rcar_du_platform_driver = {
 
 static int __init rcar_du_init(void)
 {
+	if (drm_firmware_drivers_only())
+		return -ENODEV;
+
 	rcar_du_of_init(rcar_du_of_table);
 
 	return platform_driver_register(&rcar_du_platform_driver);