drm/i915: Extract checking for backing struct pages to a helper

Currently to see if an object is backed by struct pages (as opposed to
being a simple pointer to stolen memory, for example) we do a manual
check on the obj->ops->flags. This is quite shouty and before adding
more checks in future, we should make it a bit calmer.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466431552-17860-1-git-send-email-chris@chris-wilson.co.uk