Copy kernel DRM modules to initramfs
osk-sdl requires DRM support from the kernel in the initramfs, but it does not copy the kernel DRM modules needed. Plymouth's initramfs hook does, so osk-sdl currently only works if plymouth is also installed.
Without those modules, SDL errors out saying that the kmsdrm backend is not available.
Plymouth copies DRM modules here: https://sources.debian.org/src/plymouth/22.02.122-3/debian/local/plymouth.hook/#L148
(osk-sdl does copy userpace DRI libraries, but the problem is the missing DRM support in the kernel due to missing kernel modules: https://salsa.debian.org/DebianOnMobile-team/osk-sdl/-/blob/debian/master/debian/initramfs/hooks/osk-sdl#L19)