drm/fsl-dcu: handle missing panel gracefully

If the device tree property fsl,panel is missing, drm_panel_attach
is called with a NULL pointer as first argument. Having a panel is
basically mandatory since RGB is the only supported connector.
Check if a panel node has been found, return -ENODEV and cleanup
otherwise.

Signed-off-by: Stefan Agner <stefan@agner.ch>