ACPI: thinkpad-acpi: add brightness_force parameter

Add a "brightness_enable" module parameter that allows the local admin to
force the backlight support to not be enabled.

It can also be used to force the backlight support to be enabled, but that
is currently a no-op as the backlight support is enabled by default when
available.  This will be changed by a different patch.

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