Alert devd when acpi_video brightness changes

On my Dell Latitude 7390 laptop, the brightness hotkeys
(Fn+<up/down arrow>) send ACPI notifications which acpi_video
handles by adjusting its brightness setting; but ACPI does not
actually do anything with the backlight.

Announcing brightness changes via devd makes it possible to close
the loop by triggering the intel_backlight utility to perform the
required backlight adjustment.

Reviewed by: imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D24424