i965/gs: Create brw_context::gs structure to track GS program state.

v2: Change name from "vec4_gs" to simply "gs".

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>

src/mesa/drivers/dri/i965/brw_context.h

@@ -1171,6 +1171,11 @@ struct brw_context
       struct brw_vs_prog_data *prog_data;
    } vs;
 
+   struct {
+      struct brw_stage_state base;
+      struct brw_gs_prog_data *prog_data;
+   } gs;
+
    struct {
       struct brw_ff_gs_prog_data *prog_data;