etnaviv: add set_stream_output_targets(..) stub
When running gles3 deqp's with ETNA_MESA_DEBUG=deqp we fake streamout support. CSO thinks that streamout is supported and calls ctx->pipe->set_stream_output_targets(..) in cso_destroy_context(..) which results in a null-pointer access.
Add a stub to make development easier.
Signed-off-by: Christian Gmeiner christian.gmeiner@gmail.com