drm/mgag200: Remove unused fields from struct mga_device

The fields mode_info, num_crtcs and mode in struct mga_device serve
no purpose. Remove them.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: John Donnelly <John.p.donnelly@oracle.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200507090315.21274-6-tzimmermann@suse.de