drm/i915/gen8: Pass sg_iter through pte inserts

As a step towards implementing 4 levels, while not discarding the
existing pte insert functions, we need to pass the sg_iter through.
The current function understands to the page directory granularity.
An object's pages may span the page directory, and so using the iter
directly as we write the PTEs allows the iterator to stay coherent
through a VMA insert operation spanning multiple page table levels.

v2: Rebase after s/page_tables/page_table/.
v3: Rebase after Mika's ppgtt cleanup / scratch merge patch series;
updated commit message (s/map/insert).
v4: Rebase.

Reviewed-by: Akash Goel <akash.goel@intel.com> (v3)
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Michel Thierry <michel.thierry@intel.com> (v2+)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>