drm/i915: Assert that the partial VMA fits within the object

When creating a partial VMA assert that it first fits with the parent
object, and that if it covers the whole of the parent a normal view was
created instead.

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/20161223145804.6605-5-chris@chris-wilson.co.uk