Ah, that is good.I just tried it with a fresh 4.19.63, straight from the letux git, using letux_lpae_defconfig with CONFIG_DRM_LEGACY enabled manually, and that also works fine.
(meaning it runs gles1test1)
It allows us to make a quick test if you can provide me this uImage and the kernel modules (incl. the pvrsrvkm) through some download.
Then we can decide once for ever if is kernel or kernel build related or some user-space issue on my side.