drm/sun4i: tcon: Get the connector from the encoder in RGB setup

Features such as dithering and pixel data edge configuration currently
rely on the panel registered with the TCON driver. However, bridges are
also supported in addition to panels for RGB setup.

Instead of retrieving the connector from the panel, get it from the
encoder with the dedicated helper.

Even in the case of bridges, the connector is registered with the
encoder from our driver and is accessible when iterating connectors.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181107181843.27628-3-contact@paulk.fr