drm: rcar-du: Disable fbdev emulation when no connector is present

fbdev emulation requires at least one connector, and will fail to
initialize if no connector has been successfully instantiated. Disable
it in that case and print an informational message instead of failing
probe with a confusing fbdev emulation error message.

It could be argued that probe should fail when no connector is present,
but the DU could still be useful in that case with the to-be-implemented
memory write-back support.

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