drm/fsl-dcu: remove unneeded 'ret' assignment

When devm_kzalloc() fails there is no need to assign an error code
to the 'ret' variable as it will not be used after jumping to the
'err_node_put' label, so just remove the assignment.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Stefan Agner <stefan@agner.ch>