gallium/util: silence clang warning in blitter code

Silence "warning: comparison of constant 4294967295 with expression
of type 'ubyte'".

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

src/gallium/auxiliary/util/u_blitter.c

@@ -659,7 +659,7 @@ void util_blitter_restore_fragment_states(struct blitter_context *blitter)
 
 static void blitter_check_saved_fb_state(MAYBE_UNUSED struct blitter_context_priv *ctx)
 {
-   assert(ctx->base.saved_fb_state.nr_cbufs != ~0u);
+   assert(ctx->base.saved_fb_state.nr_cbufs != (ubyte) ~0);
 }
 
 static void blitter_disable_render_cond(struct blitter_context_priv *ctx)