drm/xen: 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>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211217003752.3946210-38-javierm@redhat.com

drivers/gpu/drm/xen/xen_drm_front.c

@@ -495,6 +495,9 @@ static int xen_drm_drv_init(struct xen_drm_front_info *front_info)
 	struct drm_device *drm_dev;
 	int ret;
 
+	if (drm_firmware_drivers_only())
+		return -ENODEV;
+
 	DRM_INFO("Creating %s\n", xen_drm_driver.desc);
 
 	drm_info = kzalloc(sizeof(*drm_info), GFP_KERNEL);