drm/mgag200: remove unused driver_private access

gem_bo->driver_private is never read by mgag200 nor DRM core. No need to
set it. Besides, drm core clears it during setup, anyway.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>