tgsi: add support for geometry shader streams.

tgsi: add support for geometry shader streams.

This adds support to retrieve the primitive counts
for each stream, along with the offset for each
primitive into the output array.

It also adds support for parsing the stream argument
to the emit and end instructions.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>