drm/omap: keep ref to old_fb

We store the fb being page-flipped to 'old_fb' field, but we don't
increase the ref count of the fb when doing that. While I am not
sure if it can cause problem in practice, it's still safer to keep a ref
when storing a pointer to a fb.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>