intel_idle: Fix max_cstate for processor models without C-state tables

Currently intel_idle driver gets the c-state information from ACPI
_CST if the processor model is not recognized by it. However the
c-state in _CST starts with index 1 which is different from the
index in intel_idle driver's internal c-state table.

While intel_idle_max_cstate_reached() was previously introduced to
deal with intel_idle driver's internal c-state table, re-using
this function directly on _CST is incorrect.

Fix this by subtracting 1 from the index when checking max_cstate
in the _CST case.

For example, append intel_idle.max_cstate=1 in boot command line,
Before the patch:
grep . /sys/devices/system/cpu/cpu0/cpuidle/state*/name
POLL
After the patch:
grep . /sys/devices/system/cpu/cpu0/cpuidle/state*/name
/sys/devices/system/cpu/cpu0/cpuidle/state0/name:POLL
/sys/devices/system/cpu/cpu0/cpuidle/state1/name:C1_ACPI

Fixes: 18734958

 ("intel_idle: Use ACPI _CST for processor models without C-state tables")
Reported-by: Pengfei Xu <pengfei.xu@intel.com>
Cc: 5.6+ <stable@vger.kernel.org> # 5.6+
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>