qxl: Mark some internal functions as static

They are not used outside of their respective source file

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>
Message-id: 20161108091209.25568-2-cfergeau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>