drm/gm12u320: Don't use drm_device->dev_private

Upcasting using a container_of macro is more typesafe, faster and
easier for the compiler to optimize.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-23-daniel.vetter@ffwll.ch