simple-dmabuf-drm: 0 is a valid fd (freedreno)

Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

clients/simple-dmabuf-drm.c

@@ -244,10 +244,7 @@ static int
 fd_bo_export_to_prime(struct buffer *buf)
 {
 	buf->dmabuf_fd = fd_bo_dmabuf(buf->fd_bo);
-	if (buf->dmabuf_fd > 0)
-		return 0;
-
-	return 1;
+	return buf->dmabuf_fd < 0;
 }
 
 static int