fbdev: move udlfb out of staging.

udlfb has undergone a fair bit of cleanup recently and is effectively at
the point where it can be liberated from staging purgatory and promoted
to a real driver.

The outstanding cleanups are all minor, with some of them dependent on
drivers/video headers, so these will be done incrementally from udlfb's
new home.

Requested-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>