drm/i2c: adv7511: Convert to drm_bridge

We don't want to use the old i2c slave encoder interface anymore.

Remove that and make the i2c driver create a drm_bridge entity instead.
Converting to bridges helps because the kms drivers don't need to
exract encoder slave ops from this driver and use it within their
own encoder/connector ops.

The driver now creates its own connector when a kms driver attaches
itself to the bridge. Therefore, kms drivers don't need to create
their own connectors anymore.

The old encoder slave ops are now used by the new bridge and connector
entities.

The of_node member in drm_bridge is accessible only when CONFIG_OF is
enabled. The driver anyway only works only when OF is available. Make
the driver depend on OF in its Kconfig.

Signed-off-by: Archit Taneja <architt@codeaurora.org>