sony-laptop: brightness regression fix

After commit 540b8bb9:

  sony-laptop: fingers off backlight if video.ko is serving this functionality

I can't set brightness on my sony laptop (nothing in /sys/class/backlight).
dmesg says "sony-laptop: Sony: Brightness ignored, must be controlled by ACPI
video driver".

The function acpi_video_backlight_support returns 0 if we should use the
vendor-specific backlight support, while non-0 if the ACPI generic should
be used. Because of this, the check introduced by the said commit appears
reversed.

Signed-off-by: Alessandro Guido <ag@alessandroguido.name>
Signed-off-by: Len Brown <len.brown@intel.com>