ACPI / cpuidle: Merge acpi_idle_enter_c1() and acpi_idle_enter_simple()

acpi_idle_enter_c1() and acpi_idle_enter_simple() are close enough to
each other that they can be merged into one function which reduces
duplication of code quite a bit.

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