Set up an handler for ACPI events and respond to brightness change
requests from the system BIOS.
v2: fix notification when using device-specific command codes
(tested by Pali Rohár <pali.rohar@gmail.com>); cache the encoder
controlling the backlight during the initialization to avoid searching
it every time (suggested by Alex Deucher).
v3: whitespace fixes (Alex Deucher).
Signed-off-by: NLuca Tettamanti <kronos.it@gmail.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>