drm/i915/selftests: Fix inconsistent IS_ERR and PTR_ERR

Fix inconsistent IS_ERR and PTR_ERR in live_timeslice_nopreempt().

The proper pointer to be passed as argument to PTR_ERR() is ce.

This bug was detected with the help of Coccinelle.

Fixes: b72f02d7 ("drm/i915/gt: Prevent timeslicing into unpreemptable requests")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
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/20200616145452.GA25291@embeddedor