draw: don't use clipvertex output if user plane clipping is disabled

draw: don't use clipvertex output if user plane clipping is disabled

The non-llvm path made sure that both clip and pre_clip_pos point to the data
output by position, not clipvertex, if user based clipping is disabled.
However, the llvm path did not, which apparently led to failures if
gl_ClipVertex was written but user plane clipping not enabled (bug 80183).
Why I have no idea really, but just make it match the non-llvm behavior...

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