drm/mgag200: Remove HW cursor

The HW cursor of Matrox G200 cards only supports a 16-color palette
format. Univeral planes require at least ARGB or a similar component-
based format, so remove the HW cursor.

Alternatively, the driver could dither a cursor image from ARGB to
16 colors. But this does not produce pleasent-looking results in
general, so it's useless for modern compositors.

Without HW support, compositors will use software rendering.

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/20200515083233.32036-2-tzimmermann@suse.de