draw: fix no position output in non-llvm pipeline.

draw: fix no position output in non-llvm pipeline.

It seems easiest (and best) if we simply skip all the later stages
(after stream output).
(This is different to the llvm case at least for now where we will
simply try to render garbage, though both behaviors should be correct.)
Fixes piglit glsl-1.40-tf-no-position with softpipe.

Reviewed-by: Brian Paul <brianp@vmware.com>