drm/rockchip: use for_each_endpoint_of_node macro, drop endpoint reference on break

Using the for_each_... macro should make the code a bit shorter and
easier to read. Also, when breaking out of the loop, the endpoint node
reference count needs to be decremented.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>