drm: rcar-du: Clarify error message when encoder initialization fails

A failure to initialize an encoder currently prints an error message in
the kernel log without mentioning which encoder failed to initialize. To
help debugging initialization issues print the encoder DT node name.

This requires moving the error message to the rcar_du_encoders_init_one
function and refactoring it slightly.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>