drm/i915/gvt: Refine error handling in dispatch_workload

When an error occurs in dispatch_workload, this patch is to do the
proper cleanup and rollback to the original states before the workload
is abandoned.

v2:
- split the mixed several error paths for better review. (Zhenyu)

v3:
- original PTR_ERR(cs) is good and code cleanup. (Zhenyu)

v4:
- reuse the existing i915_add_request for error handling. (Zhenyu)

v5:
- remove the duplicate error handling release_shadow_wa_ctx and
  move the engine->context_unpin upper. (Zhenyu)

v6:
- keep the old label "out". (Zhenyu)
Signed-off-by: fred gao <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/scheduler.c

@@ -342,8 +342,10 @@ static int dispatch_workload(struct intel_vgpu_workload *workload)
 
 	if (workload->prepare) {
 		ret = workload->prepare(workload);
-		if (ret)
+		if (ret) {
+			engine->context_unpin(engine, shadow_ctx);
 			goto out;
+		}
 	}
 
 out: