drm/i915/gtt: Wrap page_table with page_directory

The page directory extends the page table with the shadow entries. Make
the page directory struct embed the page table for easier code reuse.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712094327.24437-1-chris@chris-wilson.co.uk