drm/mgag200: Don't pass flags to drm_dev_register()

The flags argument is only relevant for UMS drivers. Pass 0.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702075642.27834-2-tzimmermann@suse.de