st/nine: Do not use scratch for face register

Scratch registers are reused every instructions.
Since vFace is reused, a new temporary register
should be used.

Fixes: https://github.com/iXit/Mesa-3D/issues/311



Signed-off-by: Axel Davy <davyaxel0@gmail.com>

CC: "17.3 18.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit d8998267)