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>
Signed-off-by: Axel Davy <axel.davy@ens.fr>