powerpc/powernv: set power_save func after the idle states are initialized

pnv_init_idle_states() discovers supported idle states from the
device tree and does the required initialization. Set power_save
function pointer only after this initialization is done

Otherwise on machines which don't support nap, eg. Power9, the kernel
will crash when it tries to nap.

Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>