drm/imx: parallel-display: use correct connector enum

The imx parrallel display exposes a, well, display parallel interface.
Use the correct enum, instead of disguising the thing as a VGA connector.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>