drm/i915/gvt: use hrtimer replace delayed_work in scheduler

Currently the scheduler is triggered by delayed_work, which doesn't
provide precision at microsecond level. Move to hrtimer instead for
more accurate control.

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>