sna: Hook up a backlight udev monitor for external changes

sna: Hook up a backlight udev monitor for external changes

Changes to the backlights are notified through uevents. Hooking up a
udev monitor to listen out for external changes to the backlight (e.g.
through ACPI function keys, or by the user writing to
/sys/class/backlight directly) is easier than enabling polling on the
backlight sysfs file using X's select() mechanism.

Since we listen to backlight changes, we have to be careful not to
confuse the side-effects of disabling connectors (which may cause either
ourselves or the kernel to turn off the backlight) with the user value.

Many thanks to Alexander Mezin for the suggestion to use udev for
tracking the notifications for external changes to the backlight.

Reported-by: Alexander Mezin <mezin.alexander@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79699
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>