drm/i915: introduce gtt_pte_t

This will make the calculations of size easier to read instead of just
assuming uint32_t everywhere.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>