drm/i915/selftests: Fix wrong return value of perf_series_engines()

If intel context create failed, the perf_series_engines() will return 0
rather than error, because we doesn't initialize the return value.

Fixes: cbfd3a0c ("drm/i915/selftests: Add request throughput measurement to perf")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116144112.3673011-1-zhangxiaoxu5@huawei.com

drivers/gpu/drm/i915/selftests/i915_request.c

@@ -2467,8 +2467,10 @@ static int perf_series_engines(void *arg)
 		struct intel_context *ce;
 
 		ce = intel_context_create(engine);
-		if (IS_ERR(ce))
+		if (IS_ERR(ce)) {
+			err = PTR_ERR(ce);
 			goto out;
+		}
 
 		err = intel_context_pin(ce);
 		if (err) {