dt-bindings: fsl-imx-drm: Remove incorrect "@di0" usage

Improve the binding example by removing the '@di0' notation, which
fixes the following build warning:

Warning (unit_address_vs_reg): Node /display@di0 has a unit name, but
no reg property

Signed-off-by: Marco Franchi <marco.franchi@nxp.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>