fbdev: efifb: add dev->remove() callback

If x86-sysfb platform-devices are removed from a system, we should
properly unload efifb. Otherwise, we end up releasing the parent while our
efi framebuffer is still running. This currently works just fine, but will
cause problems on handover to real hw. So add the ->remove() callback and
unregister efifb.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>