spapr: Clean up DRC set_isolation_state() path

There are substantial differences in the various paths through
set_isolation_state(), both for setting to ISOLATED versus UNISOLATED
state and for logical versus physical DRCs.

So, split the set_isolation_state() method into isolate() and unisolate()
methods, and give it different implementations for the two DRC types.

Factor some minimal common checks, including for valid indicator values
(which we weren't previously checking) into rtas_set_isolation_state().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
