drm/exynos/dpi: use of_graph_get_endpoint_by_regs helper

This allows to remove the local of_graph_get_port_by_reg(),
of_graph_get_endpoint_by_reg(), of_get_child_by_name_reg(),
and of_graph_get_remote_port_parent() functions.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>