drm/mgag200: Don't set or clear <scroff> field during modeset

The simple pipe's disable function disables the screen by calling
mgag200_disable_screen(). The simple pipe's enable function enables the
screen by calling mgag200_enable_display().

During modeset operations the screen is off and remains off. It's only
enabled after the modeset has been completed. Therefore remove all code
that sets or clears the <scroff> field while in modeset.

The related code also modifies the <syncrst> field in SEQ0. For now, keep
this code in place.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200707082411.6583-4-tzimmermann@suse.de