x86/mwait-idle: Remove the 'preferred_cstates' parameter

Remove the 'preferred_cstates' module parameter as it is not really useful.

The parameter currently only affects Alder Lake, where it controls C1/C1E
preference, with C1E being the default. The parameter does not support any
other platform. For example, Meteor Lake has a similar C1/C1E limitation,
but the parameter does not support Meteor Lake. This indicates that the
parameter is not very useful.

Generally, independent C1 and C1E are important for server platforms where
low latency is key. However, they are not as important for client platforms,
like Alder Lake, where C1E providing better energy savings is generally
preferred.

The parameter was originally introduced for Sapphire Rapids Xeon:
da0e58c038e6 intel_idle: add 'preferred_cstates' module argument

Later it was added to Alder Lake:
d1cf8bbfed1ed ("intel_idle: Add AlderLake support")

But it was removed from Sapphire Rapids when firmware fixed the C1/C1E
limitation:
1548fac47a114 ("intel_idle: make SPR C1 and C1E be independent")

So Alder Lake is the only platform left where this parameter has any effect.
Remove this parameter to simplify the driver and reduce maintenance burden.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Link: https://patch.msgid.link/20251215111300.132803-1-dedekind1@gmail.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Origin: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git a36dc37b5672
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

docs/misc/xen-command-line.pandoc

@@ -2075,12 +2075,6 @@ compression is selected at build time from Kconfig.
 ### ple_window (Intel)
 > `= <integer>`
 
-### preferred-cstates (x86)
-> `= ( <integer> | List of ( C1 | C1E | C2 | ... )`
-
-This is a mask of C-states which are to be used preferably.  This option is
-applicable only on hardware were certain C-states are exclusive of one another.
-
 ### probe-port-aliases (x86)
 > `= <boolean>`