drm/i915: page table abstractions

When we move to dynamic page allocation, keeping page_directory and pagetabs as
separate structures will help to break actions into simpler tasks.

To help transition the code nicely there is some wasted space in gen6/7.
This will be ameliorated shortly.

Following the x86 pagetable terminology:
PDPE = struct i915_page_directory_pointer_entry.
PDE = struct i915_page_directory_entry [page_directory].
PTE = struct i915_page_table_entry [page_tables].

v2: fixed mismatches after clean-up/rebase.

v3: Clarify the names of the multiple levels of page tables (Daniel)

v4: Addressing Mika's review comments.
s/gen8_free_page_directories/gen8_free_page_directory and free the
page tables for the directory there.
In gen8_ppgtt_allocate_page_directories, do not leak previously allocated
pt in case the page_directory alloc fails.
Update error return handling in gen8_ppgtt_alloc.

v5: Do not leak pt on error in gen6_ppgtt_allocate_page_tables. (Mika)

v6: s/page_tables/page_table/. (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>