mesa/st: add vertex stream support

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nvc0/ir: remove restarts for non-0 vertex streams

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nvc0/ir: fix emitting vertex stream

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nvc0: expose 4 vertex streams, use stream ids in xfb

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>