nir: add sysval for user-clip-planes
nir: add sysval for user-clip-planes

For lowering user-clip-planes, we need a way to pass the enabled/used
user-clip-planes in to shader.

Signed-off-by: Rob Clark <6f78cdb10adbe0a7fbbf4098125a51f401c77a5f@freedesktop.org>
Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler