drm/i915/gvt: Make PTE iterator 64K entry aware

64K PTE is special, only PTE#0, PTE#16, PTE#32, ... PTE#496 are used in
the page table.

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