drm: rcar-du: Fix crash in encoder failure error path

When an encoder fails to initialize the driver prints an error message
to the kernel log. The message contains the name of the encoder's DT
node, which is NULL for internal encoders. Use the of_node_full_name()
macro to avoid dereferencing a NULL pointer, print the output number to
add more context to the error, and make sure we still own a reference to
the encoder's DT node by delaying the of_node_put() call.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>