drm/exynos/dsi: 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(), and of_get_child_by_name_reg()
functions.

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