ACPI / cpuidle: Common callback routine for entering states

Introduce a common ->enter callback routine for the ACPI cpuidle
driver, acpi_idle_enter(), which helps to reduce code complexity,
size and duplication and prevents theoretically possible failues that
an incorrect routine may be run to enter the given idle state due to
a firmware bug (eg. when _CST returns a different set of states for
each processor).

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>