util/framebuffer: add viewmask compare for fb equal

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31590>

src/gallium/auxiliary/util/u_framebuffer.c

@@ -77,6 +77,9 @@ util_framebuffer_state_equal(const struct pipe_framebuffer_state *dst,
       return false;
    }
 
+   if (dst->viewmask != src->viewmask)
+      return false;
+
    return true;
 }