drm/i915/gvt: Add 2M huge gtt support

This add 2M huge gtt support for GVTg. Unlike 64K gtt entry, we can
shadow 2M guest entry with real huge gtt. But before that, we have to
check memory physical continuous, alignment and if it is supported on
the host. We can get all supported page sizes from
intel_device_info.page_sizes.

Finally we must split the 2M page into smaller pages if we cannot
satisfy guest Huge Page.

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