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>