drm/i915/gvt: Refine shadow batch buffer

1) Use standard i915 GEM object sequence to access the shadow batch buffer.
2) Manage i915 vma life cycle to solve one FIXME.

v2:
- Refine code structure.
- Refine the usage of GEM APIs.
- Add the missing lock/unlock in release_shadow_batch_buffer.

Test on my SKL NuC.

Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>