ACPI: skip DMI power state check when ACPI disabled

This patch makes acpi_init() exit early when ACPI is disabled.
This skips a DMI check that affects ACPI power management.   The
DMI check prints a notice that is misleading when ACPI is disabled.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>