drm/i915/bxt: Enable PTE encoding

The caching options for page table entries have remained the same as
Cherryview. This patch fixes it so the right code path is taken on BXT.

v2: Fix up commit message (Mike)

Signed-off-by: Sumit Singh <sumit.k.singh@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>