drm/imx: imx-ldb: remove useless enum

Since commit 5e501ed7 ("drm/imx: imx-ldb: allow to determine bus
format from the connected panel") the enum isn't used anymore. Drop it
to cleanup the code a bit.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>