drm/mgag200: Remove type field from struct mga_device

Each model's specific code is located in a separate file. The type
field in struct mga_device is no unused. Remove it.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728124103.30159-15-tzimmermann@suse.de