drm/tilcdc: Flush flip-work workqueue before drm_flip_work_cleanup()

Flush flip-work workqueue before drm_flip_work_cleanup(). It causes a
nasty warning if there is unfinished flip-work in the queue when
drm_flip_work_cleanup() is called. The flush_workqueue() has to be
called before drm_crtc_cleanup() for unref_worker() to be able to do
its job.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>