backend-drm: schedule connector disable for detached head

Currently, if a head is detached, the entire state of the device is invalidated to make sure that the connector is disabled on the next atomic commit. Side effect of the invalid state is that all planes are disabled on the next commit. This includes planes that are used with a different head that is not part of the next atomic commit. The result is a short blanking of unrelated outputs when disabling an output.

Store the detached heads in a list on the output and disable the connectors for all heads in this list in the next atomic commit.

Signed-off-by: Michael Tretter m.tretter@pengutronix.de