meson/gbm: clean up dependencies
What does this MR do and why?
Some additional build script cleanup after !31074 (merged).
This allows building a pure-loader libgbm with something like -Degl=false -Dglx=false -Dgbm=true -Dplatforms= -Dgallium-drivers= -Dvulkan-drivers= -Dvulkan-layers=
. More dependencies could be cut from this in theory, but that would be way more invasive.