drm/i915: Use i915_vm_to_ppgtt instead of manual container_of

Looks much better without container_of everywhere.

v2:
- In i915_gem_restore_gtt_mappings too (Chris)

v3:
- Do not cause WARN by calling on non PPGTT object (Chris)

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>