broadcom/vc5: Don't forget to actually turn on stencil testing.

I had the rest of stencil state set up, but forgot to actually enable it
in the higher level configuration bits packet.

src/gallium/drivers/vc5/vc5_emit.c

@@ -265,6 +265,9 @@ vc5_emit_state(struct pipe_context *pctx)
                         } else {
                                 config.depth_test_function = PIPE_FUNC_ALWAYS;
                         }
+
+                        config.stencil_enable =
+                                vc5->zsa->base.stencil[0].enabled;
                 }
 
         }