I have just been trying and dunno as since Mesa 19.1.1 I had to compile the Master 19.1.2
Which worked but Manjaro have released 5.2 & 19.1.2 as it came out yesterday and it seems like a reversion or what I was compiling missed the deadline.
I am just going to compile mesa & drm one more time and the Master (latest) again.
PS you have to remove fbturbo driver to force panfrost
If you sudo pacman -Ss fbturbo
That will search for the package as I can never remember its full name
sudo pacman -R fbturbo whatever
reboot and the KMS driver should be loaded
pacman -S bc python-pip flex bison base-devel ncurses cmake xorg-util-macros
pip install scikit-build mako meson ninja