drm/imx: use for_each_endpoint_of_node macro in imx_drm_encoder_get_mux_id

Using the for_each_... macro should make the code bit shorter and
easier to read. This patch also properly decrements the endpoint node
reference count before returning out of the loop.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>