draw: clean up setting stream out information a bit

draw: clean up setting stream out information a bit

In particular noone is interested in the vertex count, so drop that,
and also drop the duplicated num_primitives_generated /
so.primitives_storage_needed variables in drivers. I am unable for now to figure
out if primitives_storage_needed in SO stats (used for d3d10) should
increase if SO is disabled, though the equivalent num_primitives_generated
used for OpenGL definitely should increase. In any case we were only counting
when SO is active both in softpipe and llvmpipe anyway so don't pretend there's
an independent num_primitives_generated counter which would count always.
(This means the PIPE_QUERY_PRIMITIVES_GENERATED count will still be wrong just
as before, should eventually fix this by doing either separate counting for this
query or adjust the code so it always counts this even if SO is inactive depending
on what's correct for d3d10.)

Reviewed-by: Brian Paul <brianp@vmware.com>