drm/fsl-dcu: check for clk_prepare_enable() error

clk_prepare_enable() may fail, so we should better check its return
value.

Also place the of_node_put() function right after clk_prepare_enable(),
in order to avoid calling of_node_put() twice in case clk_prepare_enable()
fails.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>