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