nir: Avoid clip/cull distance lowering multiple times.

nir: Avoid clip/cull distance lowering multiple times.

A couple places in st/nir assume that cull distances have been lowered
away, so it will need to call this lowering pass for drivers which opt
out of the GLSL IR lowering.  The Intel backend also calls this pass,
for i965 and anv.  We need to only do it once.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>