drm/i915/gvt: add basic function for weight control

This method tries to guarantee precision in second level, with the
adjustment conducted in every 100ms. At the end of each vGPU switch
calculate the sched time and subtract it from the time slice
allocated; the allocated time slice for every 100ms together with
remaining timeslice, will be used to decide how much timeslice
allocated to this vGPU in the next 100ms slice, with the end goal
to guarantee weight ratio in second level.

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>