drm/i915: Missed conversion to gtt_pte_t

commit f61c0609
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Mon Oct 22 11:44:43 2012 -0700

    drm/i915: introduce gtt_pte_t

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>