media: TDA1997x: report -ENOLINK after disconnecting HDMI source

The TD1997x chip retains vper, hper and hsper register values when the
HDMI source is disconnected. Use a different means of checking if the
link is still valid.

Signed-off-by: Krzysztof Hałasa <khalasa@piap.pl>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>