drm/i915/gvt: Let the caller choose if a shadow page should be put into hash table

As we want to re-use intel_vgpu_shadow_page in buidling scrach page table
and we don't want to put scrach page table page into hash table, a new
param is introduced to give the caller a choice to decide if a shadow page
should be put into hash table.

Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>