frontend/nine: Fix ff position_t fallback when w = 0
For post-transformed vertices, w = 0 is similar to w = 1. Replace the value to fix rcp(w). It is common for apps to pass w = 0 for position_t. cc: mesa-stable Signed-off-by:Axel Davy <davyaxel0@gmail.com> Acked-by:
David Heidelberg <david.heidelberg@collabora.com> Part-of: <mesa/mesa!18021> (cherry picked from commit b5df2056)