powerpc/8xx: fix setting of pagetable for Abatron BDI debug tool.

Commit 8c8c10b9 ("powerpc/8xx: fix handling of early NULL pointer
dereference") moved the loading of r6 earlier in the code. As some
functions are called inbetween, r6 needs to be loaded again with the
address of swapper_pg_dir in order to set PTE pointers for
the Abatron BDI.

Fixes: 8c8c10b9

 ("powerpc/8xx: fix handling of early NULL pointer dereference")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>