ACPI: PM: s2idle: Use LPS0 idle if ACPI_FADT_LOW_POWER_S0 is unset

If the PNP0D80 device is present and its _DSM appears to be valid,
there is no reason to avoid using it even if ACPI_FADT_LOW_POWER_S0
is unset in the FADT, because suspend-to-idle may be the only way to
suspend the system if S3 is not supported by the platform, so do not
return early from lps0_device_attach() in that case.

However, still check ACPI_FADT_LOW_POWER_S0 when deciding whether or
not suspend-to-idle should be the default system suspend method.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>