Current setup:
Scargarth/Styhead: Raspberry Pi 5, Qt6.8 Demo
MIPI DSI: Waveshare 8.8" with the following Device Tree Overlay
dtoverlay=vc4-kms-v3d dtoverlay=vc4-kms-dsi-waveshare-panel,8_0_inch

Problem observation:
When running, Qt6 QML UI demo, MESA-LOADER fails to load a library with the following error:
MESA-LOADER: failed to open drm-rp1-dsi: /usr/lib/dri/drm-rp1-dsi_dri.so: cannot open shared object file: No such file or directory (search paths /usr/lib/dri, suffix _dri)
Mesa uses software rendering softpipe as a fallback of V3D failure.
Qt6 UI performance is very sluggish and poor due to software rendering.

Hashes used:
meta-raspberrypi: 1aeda3f
poky: 8ce494b9cf84e769ac72b201838c76fd2ff894c9
meta-openembedded: 461d85a1831318747af5abe86da193bcde3fd9b4
meta-qt6: 5bf5b55c458c06ff8a037b2508ebe0cb4011dc76

I expected Broadcom Renderer V3D to be used as Qt backend renderer and not softpipe.

Additional info:
HDMI works well and use the correct V3D renderer since it doesn't require mesa to load rp1-dsi_dri.
The library drm-rp1-dsi_dri.so on Raspbian OS bookworm but doesn't exist anywhere else, I couldn't find any sources to build it.

Solution attempt #1:
I tried porting drm-rp1-dsi_dri.so as binary blob from Raspbian, but it had older llvm, libsensors.. etc dependencies that's harder to satisfy.
Solution attempt #1 : Fail.

Solution attempt #2:
I updated mesa to latest and greatest: 24.3.0-rc2 with the following dependencies added:
Build deps: wayland-protocols llvm python3-pyyaml python3-pyyaml-native
Runtime deps: llvm wayland-protocols
Packageconfig -= dri3
packageconfig += gallium-llvm

Solution attempt #2 was a success.
Now mesa-loader doesn't complain about missing library anymore and drm doesn't reference that library.
Qt6 Demo works with full performance, very responsive and RP1 DSI works flawlessly.

Action point:
If you deem my solution useful to upstream, I'm happy to create Pull Request.