draw: use position not clipVertex output for xyz view volume clipping

I'm pretty sure this should use position (i.e. pre_clip_pos) and not
the output from clipVertex. Albeit piglit doesn't care. It is what we
use in the clip test, and it is what every other driver does (as they
don't even have clipVertex output and lower the additional planes to
clip distances).

Reviewed-by: Brian Paul <[email protected]
Reviewed-by: Jose Fonseca <[email protected]>

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