intel_idle: further intel_idle_init_cstates_icpu() cleanup

Introduce a temporary 'state' variable for referencing the currently
processed C-state in the intel_idle_init_cstates_icpu() function.

This makes code lines shorter and easier to read.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Reviewed-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>