UPSTREAM: drm: add drm_of_encoder_active_endpoint helpers

UPSTREAM: 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.

Change-Id: I48eb7c66edb951af40085e4e388afbd5d4b2c77b
Suggested-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
(cherry pick from commit 4cacf91fcb1d7118e93caf9cb6651d7f7b56e58d)