omapdrm: omapdss_hdmi_ops: add lost_hotplug op

The CEC framework needs to know when the hotplug detect signal
disappears, since that means the CEC physical address has to be
invalidated (i.e. set to f.f.f.f).

Add a lost_hotplug op that is called when the HPD signal goes away.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>