drm/ttm: use cancel_delayed_work_sync() in ttm_bo

Make ttm_bo::ttm_bo_device_release call cancel_delayed_work_sync()
instead of calling cancel_delayed_work() followed by
flush_scheduled_work().

This is to prepare for the deprecation and removal of
flush_scheduled_work().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc:: Thomas Hellstrom <thellstrom@vmware.com>
Cc:: Dave Airlie <airlied@redhat.com>