drm/i915/gen8: Initialize page tables

Similar to gen6, while setting up a page table, make all entries of the
pt point to the scratch page before mapping; this is to be safe in case
of out of bound access or proactive prefetch.

Systems without LLC require an explicit flush.

v2: Expanded commit text and fixed indentation (Mika)

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>