drm/i915/gvt: fix use after free for workload

In the function workload_thread(), we invoke complete_current_workload()
to cleanup the just processed workload (workload will be freed there).
So we cannot access workload->req after that. This patch move
complete_current_workload() afterward.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>