drm/i915: Create a VMA for an object

In many places, we wish to store the VMA in preference to the object
itself and so being able to create the persistent VMA is useful.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-9-git-send-email-chris@chris-wilson.co.uk