intel_idle: Drop C-states redundant when PC6 is disabled

On modern Xeon platforms, such as Granite Rapids, Sierra Forest, and
Clearwater Forest, there are two flavors of requestable C6 states: C6
and C6P. C6 allows only core C6 (CC6), while C6P allows both CC6 and
package C6 (PC6). PC6 saves more power but also has a higher exit
latency, so many users disable it in BIOS.

When PC6 is disabled, C6P becomes identical to C6 — the CPU treats C6P
requests as C6 requests. Exposing both C6 and C6P to user space in this
situation is confusing: two states with the same name look different but
behave the same. It also adds unnecessary overhead to the cpuidle
subsystem, which is a fast path: the governor evaluates every registered
state on idle entry.

Drop C-states that are redundant when PC6 is disabled by marking them
with CPUIDLE_FLAG_UNUSABLE, which causes cpuidle to exclude them when
registering idle states.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Link: https://patch.msgid.link/20260425072532.358365-5-dedekind1@gmail.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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