llvmpipe: add LP_NEW_GS flag for updating vertex info

The vertex information we compute here is really dependent on the last
stage before FS. It just happened to work most of the time because new
GS tend to come with new VS and/or FS...
(The LP_NEW_GS flag was previously set but never used.)

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

src/gallium/drivers/llvmpipe/lp_state_derived.c

@@ -194,6 +194,7 @@ void llvmpipe_update_derived( struct llvmpipe_context *llvmpipe )
    /* This needs LP_NEW_RASTERIZER because of draw_prepare_shader_outputs(). */
    if (llvmpipe->dirty & (LP_NEW_RASTERIZER |
                           LP_NEW_FS |
+                          LP_NEW_GS |
                           LP_NEW_VS))
       compute_vertex_info(llvmpipe);