drm/i915/fbdev: Drain the suspend worker on retiring

drm/i915/fbdev: Drain the suspend worker on retiring

Since the suspend_work can arm itself if the console_lock() is currently
held elsewhere, simply calling flush_work() doesn't guarantee that the
work is idle upon return. To do so requires using cancel_work_sync().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468431285-28264-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>