drm/imx: parallel-display: remove unused function enc_to_imxpd()

Remove leftover container_of helper, it has been replaced by
bridge_to_imxpd().

Fixes: fe141ced ("drm/imx: pd: Use bus format/flags provided by the bridge when available")
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>