[POWERPC] Fix various offb and BootX-related issues

This patch fixes various issues with offb (the default fbdev used on
powerpc when no proper fbdev is supported). It was broken when using
BootX under some circumstances and would fail to properly get the
framebuffer base address in others.

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