drm/mgag200: Move G200SE's unique id into model-specific data

The unique revision id is only useful for G200SE devices. Store the
value in model-specific data within struct mga_device. While at it,
the patch also adds an init helper for the value.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730102844.10995-8-tzimmermann@suse.de