drm/i915/gvt: Fix shadow ctx ppgtt destroy function

Recently gvt shadow ctx create ppgtt table and this ppgtt's root
pointer is modified at workload dispatch, then we lose the original
ppgtt's root pointer, this causes the ppgtt destroy function abnormal
as it will release the wrong root table.

This patch save i915 context ppgtt root pointer at shadow
ctx creation and restore it at shadow ctx destruction.

v2: Split save and restore function (Zhenyu)

Fixes:4f15665c

("drm/i915: Add ppgtt to GVT GEM context")
Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>