draw/llvm: fix viewport transformations

draw/llvm: fix viewport transformations

This was a very serious bug. We were always doing the viewport
transformations on the first output of the vertex shader. That means
that every application that was storing position in anything but
OUT[0] was outputing untransformed vertices and had broken output
for whatever it was storing at OUT[0]. Correctly take into
consideration where the vertex position is actually stored.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>