staging: imx-drm-core: use of_graph_parse_endpoint

Using of_graph_parse_endpoint recovers the port id from an endpoint device
tree node. This just replaces an open coded read of the "reg" property.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>