pan/va: Do not insert NOPs into empty shaders
It's unnecessary and breaks the empty shader optimizations. Noticed while inspecting a trace from dEQP-GLES3.functional.color_clear.masked_scissored_rgb, which does not produce any varyings other than gl_Position in its vertex shader and hence should omit the varying shader.
Signed-off-by: Alyssa Rosenzweig alyssa@collabora.com