drm/i915/bdw: Add GTT functions

With the PTE clarifications, the bind and clear functions can now be
added for gen8.

v2: Use for_each_sg_pages in gen8_ggtt_insert_entries.

v3: Drop dev argument to pte encode functions, upstream lost it. Also
rebase on top of the scratch page movement.

v4: Rebase on top of the new address space vfuncs.

v5: Add the bool use_scratch argument to clear_range and the bool valid argument
to the PTE encode function to follow upstream changes.

v6: Add a FIXME(BDW) about the size mismatch of the readback check
that Jon Bloomfield spotted.

v7: Squash in fixup patch from Ben for the posting read to match the
64bit ptes and so shut up the WARN.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net> (v1)
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>