The data of a head is only sent when it is enabled. While the head was disabled
data might have been changed. In this case clients were not informed about this
change. A later enable change that does not also update the other data must
still lead to the propagation of this data.
Since we do not know what other data was changed while the head was disabled
just send together with an enable change all current data.
Previously, if the current configuration contains an output X which is
destroyed, its head is automatically removed. If the compositor submits
the new configuration after X was removed, the current output
configuration is incorrectly detected to be the same as the previous
one, and no done event is sent. To prevent this, we can just keep track
of whether the current configuration is dirty, i.e whether we have sent
a done event for it.