thinkpad-acpi: rework brightness support

Refactor and redesign the brightness control backend...

In order to fix bugzilla #11750...

Add a new brightness control mode: support direct NVRAM checkpointing
of the backlight level (i.e. store directly to NVRAM without the need
for UCMS calls), and use that together with the EC-based control.
Disallow UCMS+EC, thus avoiding races with the SMM firmware.

Switch the models that define HBRV (EC Brightness Value) in the DSDT
to the new mode.  These are: T40-T43, R50-R52, R50e, R51e, X31-X41.

Change the default for all other IBM ThinkPads to UCMS-only.  The
Lenovo models already default to UCMS-only.

Reported-by: Alexey Fisher <bug-track@fisher-privat.net>
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>