drm/i915: Add convenience wrappers for vma's object get/put

The VMA are unreferenced, they belong to the object and live until they
are closed. However, if we want to use the VMA as a cookie and use it to
keep the object alive, we want to hold onto a reference to the object
for the lifetime of the VMA cookie. To facilitate this, add a couple of
simple wrappers for managing the reference count on the object owning the
VMA.

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/1471254551-25805-11-git-send-email-chris@chris-wilson.co.uk