drm: use for_each_endpoint_of_node macro in drm_of_find_possible_crtcs

Using the for_each_... macro should make the code a bit shorter and
easier to read.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>