drm/gm12u320: Move driver to drm/tiny

Move the driver to the new haven for tiny DRM drivers.

Cc: Hans de Goede <hdegoede@redhat.com>
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190725105132.22545-4-noralf@tronnes.org