drm/mgag200: Don't change unrelated registers during modeset

Registers in indices below 0x18 are totally unrelated to modesetting,
so don't write 0's, or anything else into them on modeset. Most of
these registers are hardware cursor related, so this existing code
interferes with hardware cursor development.

Signed-off-by: Christopher Harvey <charvey@matrox.com>
Tested-by: Julia Lemire <jlemire@matrox.com>
Acked-by: Julia Lemire <jlemire@matrox.com>
Acked-by: Mathieu Larouche <mathieu.larouche@matrox.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>