drm/i915/gtt: Introduce init_pd_with_page

We set the page directory entries to point into a page table.
There is no gen specifics in here so make it simple and
obvious.

Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614164350.30415-3-mika.kuoppala@linux.intel.com