drm/i915/gen8: pagetable allocation rework

Start using gen8_for_each_pde macro to allocate page tables.

v2: teardown_va_range references removed.
v3: Rebase after s/page_tables/page_table/.
v4: Keep setting up page tables for all page directories in systems with
less than 4GB of memory.
v5: Also initialize the page tables. (Mika)
v6: Initialize all page tables, including the extra ones from systems
with less than 4GB of memory. (Mika)

Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
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>