drm/i915/gvt: release shadow batch buffer and wa_ctx before destroy one workload

GVT-g will shadow the privilege batch buffer and the indirect context
during command scan, move the release process into
intel_vgpu_destroy_workload() to ensure the resources are recycled
properly.

Fixes: 0cce2823

 ("drm/i915/gvt/kvmgt:Refine error handling for prepare_execlist_workload")
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>