drm: rcar-du: Hardcode encoders types to DRM_MODE_ENCODER_NONE

Unlike the connector type, the encoder type is unused by userspace. As
it is equally unused in the driver, except in a single location where
the connector type can be used instead, hardcode it to
DRM_MODE_ENCODER_NONE. This allow removing all code that tries to
determine (unsuccessfully in case a bridge is used) the encoder type.

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