drm/rockchip: 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/20211217003752.3946210-33-javierm@redhat.com

drivers/gpu/drm/rockchip/rockchip_drm_drv.c

@@ -457,6 +457,9 @@ static int __init rockchip_drm_init(void)
 {
 	int ret;
 
+	if (drm_firmware_drivers_only())
+		return -ENODEV;
+
 	num_rockchip_sub_drivers = 0;
 	ADD_ROCKCHIP_SUB_DRIVER(vop_platform_driver, CONFIG_DRM_ROCKCHIP);
 	ADD_ROCKCHIP_SUB_DRIVER(rockchip_lvds_driver,