drm/i915/gvt: dec vgpu->running_workload_num after the workload is really done

The vgpu->running_workload_num is used to determine whether a vgpu has
any workload running or not. So we should make sure the workload is
really done before we dec running_workload_num. Function
complete_current_workload is not the right place to do it, since this
function is still processing the workload. This patch move the dec op
afterward.

v2: move dec op before wake_up(&scheduler->workload_complete_wq) (Min He)

Signed-off-by: Changbin Du <changbin.du@intel.com>
Reviewed-by: Min He <min.he@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>