After reading SDDM fails to start on Pinebook Pro , I replaced libdrm-pinebookpro with libdrm (ie install libdrm and choose to remove libdrm-pinebookpro when asked to resolve the conflict).
Then, here is what I (and chatGPT) did to get my system back :
For some reason, GLX was broken so
sudo pacman -Syu
sudo pacman -S --overwrite="*" mesa mesa-utils libdrm libglvnd
sudo pacman -S libx11 libxcb xcb-util xcb-util-image xcb-util-keysyms xcb-util-renderutil xcb-util-wm xcb-util-cursor libxkbcommon-x11
sudo pacman -Qkk mesa
in order for glxinfo | grep “OpenGL renderer"to mention Panfrost, which it didn’t.