Make displaylink.c compatible with xorg-server 1.16.99.901 first RC for X server 1.17

os: Remove deprecated malloc/free wrappers
http://cgit.freedesktop.org/xorg/xserver/commit/?id=cad9b053d52f62432dfd70e42e0240de77027cae

src/displaylink.c: xnfcalloc replaced with XNFcalloc
                   xfree replaced with free
                   xalloc replaced with alloc

Signed-off-by: Xerxes Rånby <xerxes@gudinna.com>