drm/mgag200: Switch I2C code to managed cleanup

Store the I2C state within struct mga_device and switch I2C to
managed release. Simplifies the related code and lets us remove
mga_connector_destroy().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220516134343.6085-5-tzimmermann@suse.de