gallium/util: add a helper util_create_shader_from_text, cleanup some code

It's exposed, because I plan to use it elsewhere later.
st/nine: rework ATI1 ATI2 handling

Signed-off-by: Axel Davy <axel.davy@ens.fr>
st/nine: Fix bad logic in nine_state

The clip state was reset everytime, incurring an overhead.

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