drm/i915: Bump priority of clean up work

We require that we keep the list of outstanding work short so that we do
not "leak" memory while pageflipping under stress. However that system
stress may delay kernel workers virtually indefinitely, which incurs the
pageflips stall and eventually hit a timeout waiting for the cleanup.

Try to combat CPU starvation of our short-lived cleanup workers by
switching to a high priority workqueue.

Testcase: igt/kms_cursor_legacy/all-pipes-torture-move
References: https://bugs.freedesktop.org/show_bug.cgi?id=107122


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180712115729.3506-1-chris@chris-wilson.co.uk