powerpc/8xx: Move DTLB perf handling closer.

Now that space have been freed next to the DTLB miss handler,
it's associated DTLB perf handling can be brought back in
the same place.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/97f48cc1a2ea6b895bfac0752cbe59deaf2eecda.1589866984.git.christophe.leroy@csgroup.eu