drm/i915/gvt: add sprite plane flip done support.

* Add flip done event support for sprite plane on SKL platform.
* Fix bug #1452, "Call Trace:handle_default_event_virt+0xef/0x100
[i915]" while booting up guest.

Signed-off-by: Xu Han <xu.han@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>