intel_idle: refactor state->enter manipulation into its own function

Since the 6.4 kernel, the logic for updating a state's enter method
based on "environmental conditions" (command line options, cpu sidechannel
workarounds etc etc) has gotten pretty complex.
This patch refactors this into a seperate small, self contained function
(no behavior changes) for improved readability and to make future
changes to this logic easier to do and understand.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>