st/nine: Fix bad tracking of bound vs textures

st/nine: Fix bad tracking of bound vs textures

An incorrect formula was used to compute bound_samplers_mask_vs.
Since s is above always 8 for vs and the variable is encoded on 8 bits,
it was always 0.
This resulted in commiting the samplers every call when
there was at least one texture read in the vs shader.

Signed-off-by: Axel Davy <davyaxel0@gmail.com>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>