debian/local/plymouth.hook: Avoid adding Panfrost to the initramfs

The Plymouth initramfs-tools hook adds the Panfrost kernel module to the initramfs, but that fails to probe without a devfreq governor and makes hardware-accelerated graphics unavailable until the module is removed and re-probed. Plymouth actually works fine without Panfrost in the initramfs, so let's not add it to the initramfs.

Tested on an RK3399 chromebook and an AMD64 laptop with amdgpu graphics. Also see Debian bug #1001039 about the module dependency issue. I tried alternative solution in initramfs-tools but not adding the module sounded better afterwards.