BSD implements a single backlight controller interface and doesn't need
the sysfs searching of Linux. Just report that there is no per-device
backlight to fallback to using the ioctl (and in the process avoid
calling undefined functions inside the BSD block).

Reported-by: David Shao <davshao@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99833