drm/i915/gvt: Use the returned VMA to provide the virtual address

The purpose of returning the just-pinned VMA is so that we can use the
information within, like its address. Also it should be tracked and used
as the cookie to unpin...

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