glsl/nir: Only claim to handle intrinsic functions

glsl/nir: Only claim to handle intrinsic functions

Non-intrinsic function handling has never actually been tested and
probably doesn't work.  Just get rid of it for now.  We can always add
it back in later if it's useful.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>