ACPI / PM: Call acpi_save_state_mem() right before low-level suspend

Since acpi_save_state_mem() is only called by acpi_suspend_enter()
if the target sleep state is S3, it's better to call it under the
switch (acpi_state), right before do_suspend_lowlevel().

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>