intel_idle: clean up BYT/CHT auto demotion disable

Bay Trail (BYT) and Cherry Trail (CHT) platforms have a very specific way
of disabling auto-demotion via specific MSR bits. Clean up the code so that
BYT/CHT-specifics do not show up in the common 'struct idle_cpu' data
structure.

Remove the 'byt_auto_demotion_disable_flag' flag from 'struct idle_cpu',
because a better coding pattern is to avoid very case-specific fields like
'bool byt_auto_demotion_disable_flag' in a common data structure, which is
used for all platforms, not only BYT/CHT. The code is just more readable
when common data structures contain only commonly used fields.

Instead, match BYT/CHT in the 'intel_idle_init_cstates_icpu()' function,
and introduce a small helper to take care of BYT/CHT auto-demotion. This
is consistent with how platform-specific things are done for other
platforms.

No intended functional changes, compile-tested only.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Link: https://patch.msgid.link/20250210071253.2991030-1-dedekind1@gmail.com


Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>