drm/mgag200: Set MISC memory flags in mm init code

The modesetting code initialized several memory-related flags in the
MISC register. Move this code to MM initialization.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730102844.10995-6-tzimmermann@suse.de