Allow update function to not be set in wl_display_get_fd

Ignore previous patch, here's the correct version.

From 4e1bedaaf05b576f5191f8fe3a34904ab9707414 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Samuel=20R=C3=B8dal?= <samuel.rodal@nokia.com>
Date: Mon, 27 Feb 2012 15:17:20 +0100
Subject: [PATCH] Allow update function to not be set in wl_display_get_fd

The same check is done in connection_update, and now with
wl_display_flush() there's less need for the client to need to know the
connection mask.