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. Disabling the planes of unrelated outputs causes a blanking
of these outputs until output is repainted and the plane is reenabled.

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>