drm/mgag200: Update mode registers after plane registers

Setting the plane registers first and the mode registers afterwards
reproduces the sequence used by atomic helpers. Done in preparation
of switching to simple KMS helpers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: John Donnelly <John.p.donnelly@oracle.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200515083233.32036-7-tzimmermann@suse.de