drm/gma500: Make oaktrail lvds use ddc adapter from drm_connector

We're moving all uses of ddc_bus to drm_connector where they belong.
The initialization of the gma_i2c_chan for Oaktrail is a bit backwards
so it required improvements. Also cleanup the error handling in
oaktrail_lvds_init(). Since this is the last user of
gma_encoder->ddc_bus we can remove it.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220601092311.22648-8-patrik.r.jakobsson@gmail.com