drm/i915/selftests: Fix return value check in live_breadcrumbs_smoketest()

drm/i915/selftests: Fix return value check in live_breadcrumbs_smoketest()

In case of error, the function live_context() returns ERR_PTR() and never
returns NULL. The NULL test in the return value check should be replaced
with IS_ERR().

Fixes: 52c0fdb25c7c ("drm/i915: Replace global breadcrumbs with per-context interrupt tracking")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/33c46ef24cd547d0ad21dc106441491a@intel.com
[tursulin: Wrap commit text, fix Fixes: tag.]
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>