drm/i915/gvt: add some statistic routine for scheduler

Add some statistic routine to collect the time when vGPU is
scheduled in/out and the time of the last ctx submission.

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