anv/entrypoints: don't go using wayland/xcb unless they are configured

anv/entrypoints: don't go using wayland/xcb unless they are configured

The fix in:
anv: let anv_entrypoints_gen.py generate proper Wayland/Xcb guards

breaks things if wayland headers aren't installed.

Separate things out properly to avoid that problem.

[airlied: fixed up to put in pre-existing sections].
Reported-by: Arjan van de Ven
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>