drm/i915/gvt: Add shadow context descriptor updating

The current context logic only updates the descriptor of context when
it's being pinned to graphics memory space. But this cannot satisfy the
requirement of shadow context. The addressing mode of the pinned shadow
context descriptor may be changed according to the guest addressing mode.
And this won't be updated, as the already pinned shadow context has no
chance to update its descriptor. And this will lead to GPU hang issue,
as shadow context is used with wrong descriptor. This patch fixes this
issue by letting the pinned shadow context descriptor update its
addressing mode on demand.

This patch fixes GPU HANG issue which happends after changing the
grub parameter i915.enable_ppgtt form 0x01 to 0x03 or vice versa and
then rebooting the guest.

Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Signed-off-by: Kechen Lu <kechen.lu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>