drm/i915/gvt: Fix vGPU sched timeslice calculation warning

Fix below warning by using proper ktime helper to calculate timeslice.

CHECK   drivers/gpu/drm/i915//gvt/sched_policy.c
drivers/gpu/drm/i915//gvt/sched_policy.c:108 gvt_balance_timeslice() debug: sval_binop_signed: invalid divide LLONG_MIN/-1
drivers/gpu/drm/i915//gvt/sched_policy.c:108 gvt_balance_timeslice() debug: sval_binop_signed: invalid divide LLONG_MIN/-1

Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>