drm/nouveau/fbcon: fix module unload when fbcon init has failed for some reason

Stale pointer was tripping up the unload path.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_fbcon.c

@@ -590,6 +590,7 @@ fini:
 	drm_fb_helper_fini(&fbcon->helper);
 free:
 	kfree(fbcon);
+	drm->fbcon = NULL;
 	return ret;
 }