st/xorg: handle updates to DamageUnregister API

st/xorg: handle updates to DamageUnregister API

xserver 1.14.99.2 simplified the DamageUnregister API, by
dropping the drawable argument.
Follow xf86-video-intel and xf86-video-vmware approach and
handle the new API by checking XORG_VERSION_CURRENT.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71110
Reported-by: Michał Górny <mgorny@gentoo.org>
Reported-by: Vinson Lee <vlee@freedesktop.org>
Tested-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>