debian: initramfs: hooks: fix for current mesa
This hook was created back when most DRI modules were hardlinks pointing to a single module. This caused duplication when adding those to the initramfs, mandating a more complex algorithm. Nowadays, those DRI modules are symlinks though, and as such can be more easily dealt with.
Simplify the dri_inst() function to reflect that, and add the recently
added GBM module as well.