drm/i915/selftests: Fix error checking for wait_var_timeout

The old wait_on_atomic_t used a custom callback to perform the
schedule(), which used my return semantics of reporting an error code on
timeout. wait_var_event_timeout() uses the schedule() return semantics
of reporting the remaining jiffies (1 if it timed out with 0 jiffies
remaining!) and 0 on failure. This semantic mismatch lead to us falsely
claiming a time out occurred.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106085
Fixes: d224985a5e31 ("sched/wait, drivers/drm: Convert wait_on_atomic_t() usage to the new wait_var_event() API")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180417170638.20550-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/intel_breadcrumbs.c

@@ -412,10 +412,11 @@ static int igt_wakeup(void *arg)
 		 * that they are ready for the next test. We wait until all
 		 * threads are complete and waiting for us (i.e. not a seqno).
 		 */
-		err = wait_var_event_timeout(&done, !atomic_read(&done), 10 * HZ);
-		if (err) {
+		if (!wait_var_event_timeout(&done,
+					    !atomic_read(&done), 10 * HZ)) {
 			pr_err("Timed out waiting for %d remaining waiters\n",
 			       atomic_read(&done));
+			err = -ETIMEDOUT;
 			break;
 		}