efifb: Disallow manual bind and unbind

Both were buggy: bind would happily scribble over a real graphics
device and unbind wouldn't destroy the framebuffer.  Hotplugging
efifb makes no sense anyway, so just disable it.

As an added benefit, we save some runtime memory.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>