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: NGustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200616145452.GA25291@embeddedor

drivers/gpu/drm/i915/gt/selftest_lrc.c

@@ -1337,7 +1337,7 @@ static int live_timeslice_nopreempt(void *arg)
 
 		ce = intel_context_create(engine);
 		if (IS_ERR(ce)) {
-			err = PTR_ERR(rq);
+			err = PTR_ERR(ce);
 			goto out_spin;
 		}