drm/i915: Flush the workqueue before draining

Trying to drain a workqueue while we may still be adding to it from
background tasks is, according to kernel/workqueue.c, verboten. So, add
a flush_workqueue() at the start of our cleanup procedure.

References: https://bugs.freedesktop.org/show_bug.cgi?id=110550


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703171913.16585-4-chris@chris-wilson.co.uk