drm/i915/gvt: control the scheduler by timeslice usage

The timeslice usage will determine vGPU whether has chance to
schedule or not at every vGPU switch checkpoint.

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>