media: tvp5150: disable output while signal not locked

To avoid short frames on stream start, keep output pins at high impedance
while we are not properly locked onto the input signal.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>