drm/mgag200: Debug-print unique revisions id on G200 SE

The behavior of MGA G200 SE depends on the rev id. Print the id when
debugging is enabled.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191204133435.17462-1-tzimmermann@suse.de