drm/gma500: Make gma_i2c_chan a subclass of i2c_adapter

This makes it easier to get at the full gma_i2c_chan when having an
i2c_adapter from eg. drm_connector->ddc.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220601092311.22648-3-patrik.r.jakobsson@gmail.com