draw/llvm: skip clipping and viewport transform if there's no position output

draw/llvm: skip clipping and viewport transform if there's no position output

With glsl 1.40 writing position is not required (useful for transform
feedback, though in fact it's still possible to rasterize such geometry
even if the results aren't too well defined).
Prevents crashes in that case. Fixes piglit glsl-1.40-tf-no-position.
Not quite sure this is 100% correct as it also skips clipdistance
clipping which could still work (but not sure if the result would
really be needed?)

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