With GLES we always set the correct binding if dual source blend
is disabled, with OpenGL we still have to do this to handle outputs
that might be optimized away.
v2: Fix comment (Yiwei Zhang)
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Yiwei Zhang <zzyiwei@chromium.org>