draw: fix memory leak introduced 7720ce32

We need to free memory allocation PrimitiveOffsets in draw_gs_destroy().
This fixes memory leak found while running piglit on windows.

Fixes: 7720ce32 ("draw: add support to tgsi paths for geometry streams. (v2)")

Tested with piglit

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 926a6a35)