drm/i915/gvt: add a NULL pointer check to avoid kernel panic

Due to the request replay, context switch interrupt may come after
gvt free the workload thus can cause a kernel NULL pointer kernel
panic. This patch will add a simple check to avoid this for a short
term.

From long term, gvt workload lifecycle doesn't match with i915 request
and need to find a proper way to manage this.

v4: simplify the NULL pointer check.
v5: add unlikely to optimize.
Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>

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

@@ -139,6 +139,9 @@ static int shadow_context_status_change(struct notifier_block *nb,
 	struct intel_vgpu_workload *workload =
 		scheduler->current_workload[req->engine->id];
 
+	if (unlikely(!workload))
+		return NOTIFY_OK;
+
 	switch (action) {
 	case INTEL_CONTEXT_SCHEDULE_IN:
 		intel_gvt_load_render_mmio(workload->vgpu,