nv50: make blending work so that zero wins in a multiplication

This matches nvc0 behavior, tested with the fbo-float-nan piglit.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Tobias Klausmann<tobias.johannes.klausmann@mni.thm.de>

src/gallium/drivers/nouveau/nv50/nv50_screen.c

@@ -772,6 +772,11 @@ nv50_screen_init_hwctx(struct nv50_screen *screen)
       PUSH_DATA (push, 0);
    }
 
+   BEGIN_NV04(push, NV50_3D(UNK0FDC), 1);
+   PUSH_DATA (push, 1);
+   BEGIN_NV04(push, NV50_3D(UNK19C0), 1);
+   PUSH_DATA (push, 1);
+
    PUSH_KICK (push);
 }