zink: dot leak dummy_buffer

Fixes: 8736ffae2e ("zink: replace unset buffer with a dummy-buffer")

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8099>

src/gallium/drivers/zink/zink_context.c

@@ -61,6 +61,7 @@ zink_context_destroy(struct pipe_context *pctx)
 
    util_blitter_destroy(ctx->blitter);
 
+   pipe_resource_reference(&ctx->dummy_buffer, NULL);
    for (unsigned i = 0; i < ARRAY_SIZE(ctx->null_buffers); i++)
       pipe_resource_reference(&ctx->null_buffers[i], NULL);