drm/i915: Do not access stolen memory directly by the CPU, even for error capture

For stolen pages, since it is verboten to access them directly on many
architectures, we have to read them through the GTT aperture. If they
are not accessible through the aperture, then we have to abort.

This was complicated by

commit 8b6124a6
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Jan 30 14:38:16 2014 +0000

    drm/i915: Don't access snooped pages through the GTT (even for error capture)

and the desire to use stolen memory for ringbuffers, contexts and
batches in the future.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>