draw: fix incorrect NIR support code

"state" contains NIR, while "vs->base.state" contains TGSI generated
from NIR. It was a typo.

This fixes the arb_vp subtest of: DRAW_USE_LLVM=1 piglit/bin/rasterpos

Fixes: df11ceaa - draw: add NIR support to draw_create_vertex_shader
Reviewed-by: Zoltán Böszörményi <zboszor@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <mesa/mesa!8046>

src/gallium/auxiliary/draw/draw_vs_exec.c

@@ -240,7 +240,7 @@ draw_create_vs_exec(struct draw_context *draw,
       }
    }
 
-   tgsi_scan_shader(state->tokens, &vs->base.info);
+   tgsi_scan_shader(vs->base.state.tokens, &vs->base.info);
 
    vs->base.state.stream_output = state->stream_output;
    vs->base.draw = draw;