[Bug 286721] graphics/drm-61-kmod, graphics/drm-66-kmod: No backlight control

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=286721

            Bug ID: 286721
           Summary: graphics/drm-61-kmod, graphics/drm-66-kmod: No
                    backlight control
           Product: Ports & Packages
           Version: Latest
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: Individual Port(s)
          Assignee: [email protected]
          Reporter: [email protected]
          Assignee: [email protected]
             Flags: maintainer-feedback?([email protected])

Controlling LCD brightness doesn't work on drm-61 and drm-66. Works on drm-515.
-CURRENT from today (4a38527105d5), but problem was noticed months ago.

Backlight will show that value changes:
% backlight 100
% backlight
brightness: 100
% backlight 50
% backlight
brightness: 49

But brightness of the internal LCD screen doesn't change.

acpi_video.ko is loaded.
sysctl hw.acpi.video.lcd0.brightness behaves same as backlight command.

HW: Lenovo ThinkPad X1 carbon, 5th (from 2017.)
# dmesg | grep ^CPU
CPU: Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz (2900.00-MHz K8-class CPU)
# pciconf -lv | grep vgapci -A4
vgapci0@pci0:0:2:0:     class=0x030000 rev=0x02 hdr=0x00 vendor=0x8086
device=0x5916 subvendor=0x17aa subdevice=0x224f
    vendor     = 'Intel Corporation'
    device     = 'HD Graphics 620'
    class      = display
    subclass   = VGA

# backlight -i
Backlight name: intel_backlight
Backlight hardware type: Panel

Files in /dev/ are the same with drm-515 and drm-61:
/dev/backlight/
/dev/backlight/backlight0
/dev/backlight/intel_backlight0
/dev/led/kbd_backlight

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