drm/imx: imx-ldb: use of_graph_get_endpoint_by_regs helper

Instead of using of_graph_get_port_by_id() to get the port and then
of_get_child_by_name() to get the first endpoint, get to the endpoint
in a single step.

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