drm/i915: Adapt workqueue to new alloc_workqueue interface

create_singlethreaded_workqueue() is being phased out for a new
concurrency managed task infrastructure.

Adapt our workqueue constructor to explicitly create a domain that only
allows the execution of a single task at any time. All the tasks are
expected to require the dev->struct_mutex, so would block concurrency of
other tasks if we allow more than a single i915 task to be run at once.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>