drm/i915/selftests: Add some missing error propagation

Add some missing error propagation in live_parallel_switch.

To avoid needlessly burdening the various backport processes, note I am
not marking it as a fix against any patches and not copying stable since
it is debug/selftests only code.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Fixes: 50d16d44 ("drm/i915/selftests: Exercise context switching in parallel")
Fixes: 6407cf53 ("drm/i915/selftests: Stop using kthread_stop()")
Link: https://patchwork.freedesktop.org/patch/msgid/20230605131135.396854-1-tvrtko.ursulin@linux.intel.com