nir,ac/nir: fix cube_face_coord

Seems it was missing the "/ ma + 0.5" and the order was swapped.

Fixes: a1a2a8df ('nir: add AMD_gcn_shader extended instructions')
Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 8671cfe2)