drm/i915/gen8: Split out mappings

When we do dynamic page table allocations for gen8, we'll need to have
more control over how and when we map page tables, similar to gen6.
In particular, DMA mappings for page directories/tables occur at allocation
time.

This patch adds the functionality and calls it at init, which should
have no functional change.

The PDPEs are still a special case for now. We'll need a function for
that in the future as well.

v2: Handle renamed unmap_and_free_page functions.
v3: Updated after teardown_va logic was removed.
v4: Rebase after s/page_tables/page_table/.
v5: No longer allocate all PDPs in GEN8+ systems with less than 4GB of
memory, and update populate_lr_context to handle this new case (proper
tracking will be added later in the patch series).
v6: Assign lrc page directory pointer addresses using a macro. (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>