drm/i915: Make clear/insert vfuncs args absolute

This patch converts insert_entries and clear_range, both functions which
are specific to the VM. These functions tend to encapsulate the gen
specific PTE writes. Passing absolute addresses to the insert_entries,
and clear_range will help make the logic clearer within the functions as
to what's going on. Currently, all callers simply do the appropriate
page shift, which IMO, ends up looking weird with an upcoming change for
the gen8 page table allocations.

Up until now, the PPGTT was a funky 2 level page table. GEN8 changes
this to look more like a 3 level page table, and to that extent we need
a significant amount more memory simply for the page tables. To address
this, the allocations will be split up in finer amounts.

v2: Replace size_t with uint64_t (Chris, Imre)

v3: Fix size in gen8_ppgtt_init (Ben)
Fix Size in i915_gem_suspend_gtt_mappings/restore (Imre)

Reviewed-by: Imre Deak <imre.deak@intel.com> (v2)
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>