Reuse crtc

When reconfiguring the outputs after a hotplug event, we can keep the old output<->CRTC mapping for the outputs that are still present and enabled, avoiding the need to reconfigure the link, which makes the screens not go black for some time. See issue #373

This is split in two commits but could be squashed into one. For how weston did it, see:

https://gitlab.freedesktop.org/wayland/weston/commit/75487c2560dfca033d8d23649914efb4199356dd https://gitlab.freedesktop.org/wayland/weston/commit/db4c7d7237ac297cfdd22b9a1d856d9999399ff2