Not having an endpoint bound in DT should not cause a failure here,
there are fallbacks. So explicitly accept a missing endpoint.

This behavior change was introduced by refactoring in drm_of parsing
code and it should not require dts changes.

In particular this fixes imx6qdl-sabreauto boards.

Link: https://lists.freedesktop.org/archives/dri-devel/2017-May/141233.html
Fixes: ebc94461 ("drm: convert drivers to use drm_of_find_panel_or_bridge")
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>