drm/i915: Use our singlethreaded wq for freeing objects

As freeing the objects require serialisation on struct_mutex, we should
prefer to use our singlethreaded driver wq that is dedicated to work
requiring struct_mutex (hence serialised).The benefit should be less
clutter on the system wq, allowing it to make progress even when the
driver/struct_mutex is heavily contended.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180115122846.15193-1-chris@chris-wilson.co.uk


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>