drm/mgag200: Clear <page> field during MM init

The modesetting code initialized the memory-related register CRTCEXT4.
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/20200730102844.10995-7-tzimmermann@suse.de