loader: remove loader_get_driver_for_fd() driver_type

loader: remove loader_get_driver_for_fd() driver_type

Reminiscent from the pre-loader days, were we had multiple instances of
the loader logic in separate places and one could build a "GALLIUM_ONLY"
version.

Since that is no longer the case and the loaders (glx/egl/gbm) do not
(and should not) require to know any classic/gallium specific we can
drop the argument and the related code.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>