st/nine: Fix setting of the shift modifier in nine_shader
It is an sint_4, but it was stored in a uint_8... The code using it was acting as if it was signed. Problem found thanks to Coverity Cc: "10.4" <mesa-stable@lists.freedesktop.org> Tested-by:David Heidelberg <david@ixit.cz> Reviewed-by:
Ilia Mirkin <imirkin@alum.mit.edu> Signed-off-by:
Axel Davy <axel.davy@ens.fr> (cherry picked from commit d52328fc)