drm/i915: Call out GEN6 PTE specificity

We can assume that the PTE layout, and size changes for future
generations. To avoid confusion with the existing GEN6 PTE typedef, give
it a GEN6_ prefix.

v2: Fixup checkpatch warning and bikeshed commit message slightly.

v3: Rebase on top of Imre's for_each_sg_pages rework.

v4: Fixup conflicts in patch series reordering.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>