drm/i915/selftests: Fix some error codes

These error paths return success instead of negative error codes as
intended.

Fixes: c92724de

 ("drm/i915/selftests: Try to detect rollback during batchbuffer preemption")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/X/xMdcewtft7+QFM@mwanda