powerpc: convert to setup_initial_init_mm()

Use setup_initial_init_mm() helper to simplify code.

Link: https://lkml.kernel.org/r/20210608083418.137226-12-wangkefeng.wang@huawei.com


Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Souptick Joarder <jrdr.linux@gmail.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>