vc4: Fix stencil writemask handling.

If the writemask doesn't compress, then we want to put in the uncompressed
writemask, not the compressed writemask failure value (all-on).

Fixes glean's stencil2 and fbo-clear-formats on stencil.

src/gallium/drivers/vc4/vc4_state.c

@@ -223,8 +223,8 @@ vc4_create_depth_stencil_alpha_state(struct pipe_context *pctx,
 
                 if (front_writemask_bits == 0xff ||
                     back_writemask_bits == 0xff) {
-                        so->stencil_uniforms[2] = (front_writemask_bits |
-                                                   (back_writemask_bits << 8));
+                        so->stencil_uniforms[2] = (front->writemask |
+                                                   (back_writemask << 8));
                 }
         }