ACPI: thinkpad-acpi: Remove firmware backlight delays for Intel ACPI IGD OpRegion

Based on analysis and a patch from Matthew Garrett <mjg59@srcf.ucam.org>.

Instruct the ThinkPad ACPI firmware to remove delays on the processing of
backlight brightness changes.  This method is present on ThinkPad
Vista-compatible BIOSes with standard ACPI backlight level control.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Yves-Alexis Perez <corsac@debian.org>
Signed-off-by: Len Brown <len.brown@intel.com>