drm/gma500: Read EDID from the correct i2c adapter

Someone made the mistake to try reading EDID from the backlight i2c
adapter. This has been wrong for a very long time but since we read out
the modes correctly on init and don't hotplug lvds it has been working
anyway. Correct this by using connector->ddc instead of
encoder->i2c_bus. Both PSB and CDV are affected but this bug.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220601092311.22648-9-patrik.r.jakobsson@gmail.com