drm/vc4: Support nomodeset

If we have nomodeset on the kernel command line we should have the
firmware framebuffer driver kept as is and not try to load the
full-blown KMS driver.

In this case, let's just register the v3d driver.
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211215095117.176435-3-maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_drv.c

@@ -357,6 +357,9 @@ static int __init vc4_drm_register(void)
 {
 	int ret;
 
+	if (drm_firmware_drivers_only())
+		return -ENODEV;
+
 	ret = platform_register_drivers(component_drivers,
 					ARRAY_SIZE(component_drivers));
 	if (ret)