drm: rcar-du: Replace drm_encoder with drm_slave_encoder

DRM slave encoders require their associated struct drm_encoder instance
to be embedded in a struct drm_slave_encoder. This makes processing
encoders regardless of their types needlessly and painfully complex in
drivers that use a mix of slave encoders and custom encoders. Such a
driver will need to either create drm_slave_encoder instances that fake
their embedded encoder instance, or to turn all drm_encoder instances
into drm_slave_encoder instances.

Between the two evils, one must choose the lesser. Use drm_slave_encoder
everywhere.

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