drm/i915/gvt: use pfn_valid for better checking

Before get the page from pfn, use pfn_valid to check if pfn
is able to translate to page structure.

Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>