drm: rcar-du: Remove i2c slave encoder interface for hdmi encoder

The hdmi output in rcar-du uses the i2c slave encoder interface to link
to the adv7511 encoder chip. The kms driver creates encoder and connector
entities that internally uses the drm_encoder_slave_funcs ops provided by
the slave encoder driver.

Change the driver such that it expects a bridge entity instead of a slave
encoder. The hdmi connector code isn't needed anymore as we expect the
adv7511 bridge driver to create/manage the connector.

Note that the kms driver still expects a connector node for hdmi to be
present in DT. This node has no connection to the connector created
by the bridge driver.

Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>