vfb: only enable if explicitly requested when compiled in

The Kconfig help for the vfb driver says:
   Do NOT enable it for normal systems! To protect the innocent, it
   has to be enabled explicitly at boot time using the kernel option
   `video=vfb:'.

This change lets the code match the description.

Support for vfb:disable is kept for backwards compatibility; vfb:off works
because it is tested at a higher level.

Note: any undefined option (e.g. vfb:enable) will also enable this driver.

The relevant code has been unchanged since before the migration to
git (2.6.12).

This patch fixes bugzilla #9310 and was the root cause behind
http://lkml.org/lkml/2008/5/31/220

.

Signed-off-by: Frans Pop <elendil@planet.nl>
Cc: Antonino A. Daplas <adaplas@gmail.com>
Acked-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>