Actually, about your question:

> And you're making sure that the X server picks up your self-built radeonsi_dri.so for glamor?

I'm compiling Mesa in my home directory with the following configure switches:

./autogen.sh --enable-gles1 --enable-gles2 --enable-glx-tls
--with-egl-platforms=x11,drm,wayland --with-dri-drivers=radeon
--with-gallium-drivers=radeonsi --enable-debug --enable-autotools
make -j8

export LD_LIBRARY_PATH=/home/diego/mesa/lib
export LIBGL_DRIVERS_PATH=/home/diego/mesa/lib/gallium

So I assume this should make the X server pick the radeonsi_dri.so from
/home/diego/mesa/lib/gallium right?

I looked at the ~/.local/share/xorg/Xorg.0.log but it's not so clear:

[   437.304] (II) modeset(0): [DRI2]   DRI driver: radeonsi
[   437.307] (II) AIGLX: Loaded and initialized radeonsi

Is there a way to verify beyond this which radeonsi driver it's using?