[Bug 267730] graphics/drm-kmod Screen artifacts with amdgpu

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=267730

            Bug ID: 267730
           Summary: graphics/drm-kmod  Screen artifacts with amdgpu
           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]
             Flags: maintainer-feedback?([email protected])
          Assignee: [email protected]

Describe the bug:
When using amdgpu, white-on-black horizontal lines intermittently flash on
right half of screen. See video attached below in Screenshots

FreeBSD version:
FreeBSD fbsd 14.0-CURRENT FreeBSD 14.0-CURRENT #0 main-n258691-24cf7a8d628: Thu
Oct 20 07:55:17 UTC 2022
[email protected]:/usr/obj/usr/src/amd64.amd64/sys/GENERIC amd64

pciconf:

vgapci0@pci0:9:0:0:     class=0x030000 rev=0xc8 hdr=0x00 vendor=0x1002
device=0x1638 subvendor=0x1043 subdevice=0x8809
    vendor     = 'Advanced Micro Devices, Inc. [AMD/ATI]'
    device     = 'Cezanne'
    class      = display
    subclass   = VGA

DRM KMOD version:
drm-510-kmod 5.10.113_7
drm-kmod 20220907_1

To Reproduce:
Install either 13.1-RELEASE, 13.1-STABLE, or 14.0, install drm-kmod package,
enable amdgpu by adding kld_list="amdgpu" to /etc/rc.conf, and log into a
console.

Screenshot:
https://user-images.githubusercontent.com/11449161/198061203-8056e97d-7a3d-4ccb-945f-fcc6423e88cb.mp4

Additional context:
It appears support for vendor=0x1002 device=0x1638 graphics card has been
added:
https://github.com/freebsd/drm-kmod/blob/5.10-lts/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c#L1102
Same issue presents on any FreeBSD version listed above, whether built from
source or installed from snapshot.
Disabling video mode using hw.vga.textmode=1 does not resolve the issue.
The issue carries over into an X session.
Notably, the issue disappears when compiling anything, whether in the console
or X.
Github issue previously opened under freebsd/drm-kmod at
https://github.com/freebsd/drm-kmod/issues/213

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