drm/atmel-hlcdc: always iterate over the first 4 output endpoints

This enables more flexible devicetrees. You can e.g. have two output
nodes where one is not enabled, without the ordering affecting things.

Prior to this patch the active nodes had to have endpoint id zero and
upwards consecutively.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180825085620.10566-4-peda@axentia.se