drm/i915/gvt: Move request alloc to dispatch_workload path only

Previously the performance is improved through the workload auditing
and shadowing ahead of vGPU scheduling, however, there is the case that
more requests are allocated in submit_context before the previous request
is added, the timeline will hold its seqno which is later.

This patch is to move the request alloc to dispatch_workload function,
where is the same place as request is added.

It will fix the issue of kernel BUG for (timeline->seqno != request->fence.seqno)
check when add_request.

Fixes: 89ea20b9 ("drm/i915/gvt: Factor out scan and shadow from workload dispatch")
Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: fred gao <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
(cherry picked from commit f2880e04)