drm/i915/selftests: Fix error checking for wait_var_timeout

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