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>