drm/i915/gvt: Double check batch buffer size after copy

Double check the end of the privilege buffer to make sure the size
of the privilege buffer remains unchanged after copy.

v4:
- Refine the commit message. (Zhenyu)
v3:
- To get the right offset of the batch buffer end cmd. (Yan)

v2:
- Use lightweight way to audit batch buffer end. (Yan)

Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>