drm: rcar-du: Improve kernel log messages when initializing encoders

Improve the debugging and error messages printing when initializing
encoders by replacing the output number by the output name, printing the
bridge OF node name, and the error code of failed operations.

While at it, move the related rcar_du_output enumeration from
rcar_du_crtc.h to rcar_du_drv.h as it's not specific to the CRTC.

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