drm/imx: parallel-display: allow to determine bus format from the connected panel

Similarly to commit 5e501ed7 ("drm/imx: imx-ldb: allow to determine
bus format from the connected panel"), if a panel is connected to the ldb
output port via the of_graph bindings, the data mapping is determined from
the display_info.bus_format field provided by the panel instead of from the
optional interface_pix_fmt device tree property.

Reported-by: Ulrich Ölmann <u.oelmann@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Gary Bisson <gary.bisson@boundarydevices.com>