[media] cec: allow specific messages even when unconfigured

The CEC specifications explicitly allows you to send poll messages and
Image/Text View On messages to a TV, even when unconfigured (i.e. there is
no hotplug signal detected). Some TVs will pull the HPD low when switching
to another input, or when going into standby, but CEC should still be
allowed to wake up such a display.

Add support for sending messages with initiator 0xf (Unregistered) and
destination 0 (TV) when no physical address is present.

This also required another change: the CEC adapter has to stay enabled as
long as 1) the CEC device is configured or 2) at least one filehandle is open
for the CEC device.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>