Comment 57 for bug 1944397

Installed hirsute desktop image, upgraded all packages and confirmed the problem:

$ uname -a
Linux rpi-4b-rev1d4-d9f5 5.11.0-1019-raspi #20-Ubuntu SMP PREEMPT Tue Sep 21 15:23:42 UTC 2021 aarch64 aarch64 aarch64 GNU/Linux
$ dmesg | grep -P 'firmware|WARN'
[ 0.116402] raspberrypi-firmware soc:firmware: Attached to firmware from 2021-02-25T12:10:40, variant start
[ 0.120417] raspberrypi-firmware soc:firmware: Firmware hash is 564e5f9b852b23a330b1764bcf0b2d022a20afd0
[ 94.323547] raspberrypi-firmware soc:firmware: Request 0x00030066 returned status 0x80000001
[ 94.332200] vc4-drm gpu: [drm] Couldn't stop firmware display driver: -22
[ 94.352626] WARNING: CPU: 3 PID: 1171 at drivers/gpu/drm/vc4/vc4_hdmi_regs.h:417 vc4_hdmi_read+0xd8/0x140 [vc4]
[ 94.708647] WARNING: CPU: 3 PID: 1171 at drivers/gpu/drm/vc4/vc4_hdmi_regs.h:445 vc4_hdmi_cec_init+0x3ac/0x540 [vc4]
[ 95.061054] WARNING: CPU: 1 PID: 1171 at drivers/gpu/drm/vc4/vc4_hdmi_regs.h:417 vc4_hdmi_read+0xd8/0x140 [vc4]
[ 95.416872] WARNING: CPU: 1 PID: 1171 at drivers/gpu/drm/vc4/vc4_hdmi_regs.h:445 vc4_hdmi_cec_init+0x398/0x540 [vc4]
[ 95.768827] WARNING: CPU: 0 PID: 323 at drivers/gpu/drm/vc4/vc4_hdmi_regs.h:417 vc4_hdmi_read+0xd8/0x140 [vc4]
[ 96.088013] WARNING: CPU: 0 PID: 323 at drivers/gpu/drm/vc4/vc4_hdmi_regs.h:445 vc4_cec_irq_handler_tx_bare+0xf8/0x140 [vc4]

Installed kernel from proposed and verified that the issue is fixed and the display comes up:
$ apt-add-repository -p proposed
$ apt install linux-raspi
$ reboot
<snip>
$ uname -a
Linux rpi-4b-rev1d4-d9f5 5.11.0-1021-raspi #22-Ubuntu SMP PREEMPT Wed Oct 6 17:30:38 UTC 2021 aarch64 aarch64 aarch64 GNU/Linux
$ dmesg | grep -P 'firmware|WARN'
$ sudo dmesg | grep -P 'firmware|WARN'
[ 0.120342] raspberrypi-firmware soc:firmware: Attached to firmware from 2021-02-25T12:10:40, variant start
[ 0.124357] raspberrypi-firmware soc:firmware: Firmware hash is 564e5f9b852b23a330b1764bcf0b2d022a20afd0