thinkpad-acpi: report brightness events when required

Report KEY_BRIGHTNESSUP and KEY_BRIGHTNESSDOWN input events when the
ThinkPad is in "passive brightness control" mode (because either we or
ACPI video touched _BCL), and ACPI video is not processing these
events by itself.

This happens only on Lenovo ThinkPads with ACPI video support, when
operating with the ACPI video driver in acpi_backlight=vendor mode.

Issuing these events is the right thing to do, and will work around
bugzilla #13368, if userspace is properly configured and actively
handles these events.

For other ThinkPads, and when ACPI video is handling brightness
changes, thinkpad-acpi will continue NOT sending KEY_BRIGHTNESS*
events by default.

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