Lower resolution modes corrupt and unusable on laptop display (functional regression in v6.4-rc6-12-ge749dd10e5f2)

Before submitting your bug report:

Brief summary of the problem:

Running lower resolution modes on a hiDPI laptop screen is a common thing, either for combining both a hiDPI laptop screen and non-hiDPI external displays, or for running non-hiDPI desktops and other software, or to increase the frame rate for gaming. This has worked for a long time until the following recent commit:

commit e749dd10e5f292061ad63d2b030194bf7d7d452c
Author: Hersen Wu <hersenxs.wu@amd.com>
Date:   Thu May 25 08:37:40 2023 -0400

    drm/amd/display: edp do not add non-edid timings
    
    [Why] most edp support only timings from edid. applying
    non-edid timings, especially those timings out of edp
    bandwidth, may damage edp.
    
    [How] do not add non-edid timings for edp.

Running xrandr before this commit gives output:

eDP connected primary 1280x800+0+0 (normal left inverted right x axis y axis) 286mm x 178mm
   2560x1600     60.00 +  40.00  
   1920x1200     60.00  
   1920x1080     60.00  
   1600x1200     60.00  
   1680x1050     60.00  
   1280x1024     60.00  
   1440x900      60.00  
   1280x800      60.00* 
   1280x720      60.00  
   1024x768      60.00  
   800x600       60.00  
   640x480       60.00  

Running xrandr after this commit shows only the 2560x1600 mode available.

The commit doesn't appear to actually disable the low-res mode fully, so it is still possible to change to the mode (e.g. video=eDP-1:1280x800 in the kernel parameters), but after the change the display will be corrupt and unusable.

Hardware description: HP Pavilion Aero Laptop 13-be0xxx/8916, BIOS F.12 04/11/2023

• CPU: AMD Ryzen 7 5800U with Radeon Graphics
• GPU: [AMD/ATI] Cezanne [Radeon Vega Series / Radeon Vega Mobile Series] [1002:1638] (rev c1)
• System Memory: 16GB
• Display(s): laptop screen 2560x1600
• Type of Display Connection: eDP

System information:

• Debian 12
• Custom kernel: v6.4-rc7-234-g547cc9be86f4
• AMD official driver version: N/A

How to reproduce the issue:

• kernel console: add video=eDP-1:1280x800 to kernel command line

• or xorg: autorandr mobile (where "mobile" is a pre-saved config for 1280x800)

Image