drm/i915: Correct a comment about the use of the workqueue.

It isn't used for the hangcheck, which does its work right from the
timer trigger, but hangcheck can lead to error state recording, which
is run off of the workqueue.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>