drm/i915/gen8: page directories rework allocation

Start using gen8_for_each_pdpe macro to allocate the page directories.

Similar to PTs, while setting up a page directory, make all entries of
the  pd point to the scratch pd before mapping (and make all its entries
point to the scratch page); this is to be safe in case of out of bound
access or  proactive prefetch. Systems without LLC require an explicit
flush.

v2: Rebased after s/free_pt_*/unmap_and_free_pt/ change.
v3: Rebased after teardown va range logic was removed.
v4: Keep setting up all page directories for systems with less than 4GB
of memory.
v5: Initialize PDs. (Mika)
v6: Initialize also the extra PDs 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>