Comment # 8 on bug 1185174 from Takashi Iwai

OK, thanks, then it's just the missing CONFIG_DRM_LEGACY that was introduced
since 5.11.  The exposure of agpgart frontend was disabled without this.

But, as this is marked as "DANGEROUS" in Kconfig

menuconfig DRM_LEGACY
        bool "Enable legacy drivers (DANGEROUS)"
        depends on DRM && MMU
        help
          Enable legacy DRI1 drivers. Those drivers expose unsafe and dangerous
          APIs to user-space, which can be used to circumvent access
          restrictions and other security measures. For backwards compatibility
          those drivers are still available, but their use is highly
          inadvisable and might harm your system.

          You are recommended to use the safe modeset-only drivers instead, and
          perform 3D emulation in user-space.

          Unless you have strong reasons to go rogue, say "N".

So, as already expected, this bug is likely WONTFIX.