drm: add drm_of_encoder_active_endpoint helpers

This patch adds a helper to parse the encoder endpoint connected to the
encoder's crtc and two helpers to return its id and port id.

This can be used to determine input mux setting from endpoint or port ids.

Suggested-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>