I use HP EliteBook 745 G5 with Ryzen 5 PRO 2500U.

Starting with the:
commit 262485a50fd4532a8d71165190adc7a0a19bcc9e (refs/bisect/bad)
Author: Anthony Koo <Anthony.Koo@amd.com>
Date:   Fri Oct 12 21:34:32 2018 -0400

    drm/amd/display: Expand dc to use 16.16 bit backlight

    [Why] We want to increase precision for backlight setting.
    But DC interface takes 8 bit backlight level value only.

    [How] DMCU already takes 16 bit backlight level.
    Expand the DC interface to take 16.16 bit value.
    Max 32 bit backlight value (0xFFFF) will represent
    max backlight (100%)

    Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
    Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
    Acked-by: Leo Li <sunpeng.li@amd.com>
    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

After resume from RAM my notebook display is all black.

Before suspend:
> cat /sys/class/backlight/amdgpu_bl0/{actual_brightness,brightness}
65535
255

After resume:
> cat /sys/class/backlight/amdgpu_bl0/{actual_brightness,brightness}
0
255

This can be workarounded by:
echo 255 > /sys/class/backlight/amdgpu_bl0/brightness
assuming one can execute that without a working display.

This problem still exists in the 5.0.0-rc2.