powerpc/pseries: Introduce dlpar_unisolate_drc()

Next patch will execute a set-indicator call in hotplug-cpu.c.

Create a dlpar_unisolate_drc() helper to avoid spreading more
rtas_set_indicator() calls outside of dlpar.c.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210416210216.380291-2-danielhb413@gmail.com