drm/i915: fix module unload since error_state rework

We need to remove the debugfs file. Regression introduce in

commit d5442303
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Apr 27 15:17:40 2012 +0200

    drm/i915: allow the existing error_state to be destroyed
Reported-and-Tested-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2032,6 +2032,8 @@ void i915_debugfs_cleanup(struct drm_minor *minor)
 				 1, minor);
 	drm_debugfs_remove_files((struct drm_info_list *) &i915_ring_stop_fops,
 				 1, minor);
+	drm_debugfs_remove_files((struct drm_info_list *) &i915_error_state_fops,
+				 1, minor);
 }
 
 #endif /* CONFIG_DEBUG_FS */