drm/i915/gvt: Use common mapping routines for shadow_bb object

We have the ability to map an object, so use it rather than opencode it
badly. Note that the object remains permanently pinned, this is poor
practise.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>