draw: make sure clipdistances work with geometry shaders

draw: make sure clipdistances work with geometry shaders

we were always fetching the info from the vertex shader, but if
geometry shader is present it should be used as the source of
that info.

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