drm/i915/gtt: Teach restore-gtt to walk the ggtt vma list not the object list

In preparation, for having non-vma objects stored inside the ggtt, to
handle restoration of the GGTT following resume, we need to walk over
the ggtt address space rebinding vma, as opposed to walking over bound
objects looking for ggtt entries.

v2: Skip objects only bound for the aliasing_ppgtt

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20180605082856.19221-1-chris@chris-wilson.co.uk