r600: add SET_NULL_SHADER macro.
r600: add SET_NULL_SHADER macro.

This is used to set a hw shader to NULL.

Signed-off-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler