www/qt6-webengine: fix running under wayland

When starting, chromium calls GpuFence::Wait() which was not implemented
on FreeBSD, on linux is calls sync_wait which does not exist on FreeBSD.

Hopefully drm sync fence fds do support polling and it seems to do the
trick.

While here tell chromium we don't have sync_fence_info() so it does not
try to go further at the risk of reaching another NOTREACH() which would
trigger a crash

PR: 287599