st/mesa: For vertex shaders, don't emit saturate when SM 3.0 is unsupported

st/mesa: For vertex shaders, don't emit saturate when SM 3.0 is unsupported

There is a bug in the current lowering pass implementation where we lower saturate
to clamp only for vertex shaders on drivers supporting SM 3.0. The correct behavior
is to actually lower to clamp only when we don't support saturate which happens
on drivers that don't support SM 3.0

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>