drm/mgag200: Switch to managed MM

The memory-management code now cleans up automatically as part of
device destruction.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605135803.19811-8-tzimmermann@suse.de