drm/i915/gvt: clear the vGPU reset logic

Releasing shadow PPGTT pages is not enough when vGPU reset, the
guest page table tracking data should has same life-cycle with
all the shadow PPGTT pages; Otherwise there is no chance to
re-shadow the PPGTT pages without free the guest page table
tracking data.

This patch clear the PPGTT reset logic and make the vGPU reset in
working order.

v2: refactor some logic to avoid code duplicated.
v3: remove useless macro and add comments from Christophe.
v4: keep reset logic in reset function.

Signed-off-by: Ping Gao <ping.a.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>