ac/nir: fix txf_ms with an offset

Seems to fix some hair artifacts in Max Payne 3:
https://github.com/daniel-schuermann/mesa/issues/76



Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Fixes: f4e499ec ('radv: add initial non-conformant radv vulkan driver')
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit a9f58af4)