powerpc/8xx: Invalidate non present TLB as early as possible

8xx sometimes need to load a invalid/non-present TLBs in
it DTLB asm handler.

These must be invalidated separaly as linux mm doesn't.

Commit 5efab4a0 was invalidating them in
arch/powerpc/mm/fault.c.
This patch does the invalidation earlier in order to free the TLB as soon as
possible. This also has the advantage of removing some 8xx specific code from
fault.c

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <scottwood@freescale.com>