i915: use alloc_ordered_workqueue() instead of explicit UNBOUND w/ max_active = 1

This is an equivalent conversion and will ease scheduled removal of
WQ_NON_REENTRANT.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>