drm/i915/selftests: Check for an already completed timeslice

With timeslice yielding on a semaphore, we may complete timeslices much
faster than we were expecting and already have yielded the stuck
request. Before complaining that timeslicing is not enabled, check that
we haven't already applied the switch.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200410081638.19893-1-chris@chris-wilson.co.uk