drm/cirrus: Fix NULL pointer dereference when registering the fbdev

cirrus_modeset_init() is initializing/registering the emulated fbdev
and, since commit c61b93fe51 ("drm/atomic: Fix remaining places where
!funcs->best_encoder is valid"), DRM internals can access/test some of
the fields in mode_config->funcs as part of the fbdev registration
process.
Make sure dev->mode_config.funcs is properly set to avoid dereferencing
a NULL pointer.

Reported-by: Mike Marshall <hubcap@omnibond.com>
Reported-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Fixes: c61b93fe51 ("drm/atomic: Fix remaining places where !funcs->best_encoder is valid")
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/cirrus/cirrus_main.c

@@ -185,14 +185,23 @@ int cirrus_driver_load(struct drm_device *dev, unsigned long flags)
 		goto out;
 	}
 
+	/*
+	 * cirrus_modeset_init() is initializing/registering the emulated fbdev
+	 * and DRM internals can access/test some of the fields in
+	 * mode_config->funcs as part of the fbdev registration process.
+	 * Make sure dev->mode_config.funcs is properly set to avoid
+	 * dereferencing a NULL pointer.
+	 * FIXME: mode_config.funcs assignment should probably be done in
+	 * cirrus_modeset_init() (that's a common pattern seen in other DRM
+	 * drivers).
+	 */
+	dev->mode_config.funcs = &cirrus_mode_funcs;
 	r = cirrus_modeset_init(cdev);
 	if (r) {
 		dev_err(&dev->pdev->dev, "Fatal error during modeset init: %d\n", r);
 		goto out;
 	}
 
-	dev->mode_config.funcs = (void *)&cirrus_mode_funcs;
-
 	return 0;
 out:
 	cirrus_driver_unload(dev);