ui: add precondition for dpy_get_ui_info()

Ensure that it only get called when dpy_ui_info_supported(). The
function should always return a result. There should be a non-null
console or active_console.

Modify the argument to be const as well.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Albert Esteve <aesteve@redhat.com>