[POWERPC] More offb/bootx fixes

There were still some issues with offb when BootX doesn't provide a
proper display node, this fixes them.  This also re-instates the
palette hacks that were disabled a couple of kernel versions ago when
I converted to the new OF parsing, and shuffles some functions around
to avoid prototypes.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>