drm/i915: Clip the partial view against the object not vma

The VMA is later clipped against the vm_area_struct before insertion of
the faulting PTE so we are free to create the partial view as we desire.
If we use the object as the extents rather than the area, this partial
can then be used for other areas.

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/20170110095633.6612-2-chris@chris-wilson.co.uk