drm/i915: Note the addition of timeslicing to the pretend scheduler

Since writing the comment that the scheduler is entirely passive, we've
added minimal timeslicing which adds the most primitive of active
elements (a timeout and reschedule).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010071434.31195-1-chris@chris-wilson.co.uk