drm/i915/bxt: map GTT as uncached

On Broxton per specification the GTT has to be mapped as uncached.
This was caught by the PTE write readback warning, which showed a
corrupted PTE value with using the current write-combine mapping.

v2:
- add comment explaining how the problem with WC mapping manifests
  (Daniel)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Antti Koskipää <antti.koskipaa@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>