powerpc/64s: Simplify POWER9 DD1 idle workaround code

The idle workaround does not need to load PACATOC, and it does not
need to be called within a nested function that requires LR to be
saved.

Load the PACATOC at entry to the idle wakeup. It does not matter which
PACA this comes from, so it's okay to call before the workaround. Then
apply the workaround to get the right PACA.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>