powerpc/64: Refactor interrupt exit irq disabling sequence

The same complicated sequence for juggling EE, RI, soft mask, and
irq tracing is repeated 3 times, tidy these up into one function.

This differs qiute a bit between sub architectures, so this makes
the ppc32 port cleaner as well.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200429062421.1675400-1-npiggin@gmail.com