drm/i915: disable wc gtt pte mappings on gen2

It doesn't work since the gtt pte range sits in the middle of the mmio
bar. We didn't notice that since both my and Chris' gen2 machines
don't support PAT and hence all wc io mapping request will
automatically be demoted to uc.

This regression has been introduce in

commit edef7e68
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Sep 14 11:57:47 2012 +0100

    agp/intel: Use a write-combining map for updating PTEs

Reported-by: Egbert Eich <eich@pdx.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=55834


Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>