Display output laggy from iGPU when operating on desktop with attaching AMD R7 430 graphic (dGPU)

[Steps to reproduce this issue]

1. Install a focal base image
2. Attach an AMD R7 430 graphic as dGPU without connecting any monitor on it.
3. Attach DP monitor on iGPU (no matter the iGPU is Intel(i915), or AMD(amdgpu)).
4. After logging it, the display shows laggy.

[Additional information]

1. doesn't see this issue if selecting 'Wayland'
2. add a config

Section "OutputClass"
 Identifier "MyRadeon"
 MatchDriver "radeon"
 Driver "modesetting"
EndSection

to fallback to use modesetting could workaround this issue.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: xserver-xorg 1:7.7+19ubuntu14
ProcVersionSignature: Ubuntu 5.10.0-1019.20-oem 5.10.18
Uname: Linux 5.10.0-1019-oem x86_64
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
Date: Fri Apr 9 16:23:42 2021
DistUpgraded: Fresh install
DistroCodename: focal
DistroVariant: ubuntu
ExtraDebuggingInterest: Yes, including running git bisection searches
GraphicsCard:
 Advanced Micro Devices, Inc. [AMD/ATI] Oland [Radeon HD 8570 / R7 240/340 / Radeon 520 OEM] [1002:6611] (rev 87) (prog-if 00 [VGA controller])
   Subsystem: Hewlett-Packard Company Oland [Radeon HD 8570 / R7 240/340 / Radeon 520 OEM] [103c:3375]
 Advanced Micro Devices, Inc. [AMD/ATI] Renoir [1002:1636] (rev da) (prog-if 00 [VGA controller])
   Subsystem: Hewlett-Packard Company Renoir [103c:872b]
InstallationDate: Installed on 2021-04-09 (0 days ago)
InstallationMedia: Ubuntu 20.04 "Focal" - Build amd64 LIVE Binary 20210324-23:53
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.10.0-1019-oem root=UUID=027c2398-aef7-43d2-a339-6a2163287914 ro quiet splash vt.handoff=7
SourcePackage: xorg
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 12/30/2020
dmi.bios.release: 2.0
dmi.bios.vendor: HP
dmi.bios.version: S09 Ver. 02.02.00
dmi.board.name: 872B
dmi.board.vendor: HP
dmi.board.version: KBC Version 09.94.00
dmi.chassis.type: 3
dmi.chassis.vendor: HP
dmi.ec.firmware.release: 9.148
dmi.modalias: dmi:bvnHP:bvrS09Ver.02.02.00:bd12/30/2020:br2.0:efr9.148:svnHP:pn:pvr:rvnHP:rn872B:rvrKBCVersion09.94.00:cvnHP:ct3:cvr:
dmi.product.family: 103C_53307F HP EliteDesk
dmi.sys.vendor: HP
version.compiz: compiz N/A
version.libdrm2: libdrm2 2.4.102-1ubuntu1~20.04.1
version.libgl1-mesa-dri: libgl1-mesa-dri 20.2.6-0ubuntu0.20.04.1
version.libgl1-mesa-glx: libgl1-mesa-glx N/A
version.xserver-xorg-core: xserver-xorg-core 2:1.20.9-2ubuntu1.2~20.04.1
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev N/A
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:19.1.0-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20200226-1
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.16-1

XorgLog.txt