drm/sun4i: tcon: Pass encoder to RGB setup function

Passing the encoder to the TCON RGB setup functions allows accessing the
connector from the encoder directly instead of relying on the panel.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181107181843.27628-2-contact@paulk.fr