drm/i915: Extract PPGTT param from page_directory alloc

drm/i915: Extract PPGTT param from page_directory alloc

This will be useful for when we move to 48b addressing, and the PDP isn't
the root of the page table structure.

v2: Rebase after changes for Gen8+ systems with less than 4GB of memory.
v3: Rebase after Mika's code review.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Michel Thierry <michel.thierry@intel.com> (v2)
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>