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

We want to use the GTT for reading back objects upon an error so that we
have exactly the information that the GPU saw. However, it is verboten
to access snoopable pages through the GTT and causes my PineView GPU to
throw a page fault instead.

This has not been a problem in the past as we only dumped ringbuffers
and batchbuffers, both of which must be not snooped. However, the
introduction of HWS page dumping leads to a read of a snooped object
through the GTT. This was introduced by

commit f3ce3821
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Jan 23 22:40:36 2014 +0000

    drm/i915: Include HW status page in error capture

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet:s/uncached/not snooped/ for one case in the commit message as
requested by Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>