drm/i915/gvt: Split ppgtt_alloc_spt into two parts

We need a interface to allocate a pure shadow page which doesn't have
a guest page associated with. Such shadow page is used to shadow 2M
huge gtt entry.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>