drm/i915: num_pd_pages/num_pd_entries isn't useful

These values are never quite useful for dynamic allocations of the page
tables. Getting rid of them will help prevent later confusion.

v2: Updated to use unmap_and_free_pd functions.
v3: Updated gen8_ppgtt_free after teardown logic was removed.
v4: Rebase after s/page_tables/page_table/.
v5: Keep allocating all page directories in GEN8+ systems with less
than 4GB of memory. Updated gen6_for_all_pdes.
v6: Prevent (harmless) out of range access in gen6_for_all_pdes.

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>