ACPI: thinkpad-acpi: prefer standard ACPI backlight level control

Newer Lenovo BIOSes support the standard ACPI backlight brightness
interface (_BCM, _BQC, _BCL).  It should be used instead of the native
thinkpad backlight brightness control interface when possible.

This patch disables the native brightness support in the driver by default
when we detect that the standard ACPI interface is available.  The local
admin can still enable it using the module parameter "brightness_enable".

Note that we need to detect the standard ACPI backlight interface only in
boxes for which we would load the native backlight interface in the first
place, and that no ThinkPad BIOS has _BCL but misses the other methods, so
the detection routines can be really simple.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>