media: atomisp: ov2680: Delay power-on till streaming is started

Move the setting of the mode to stream on, this also allows
delaying power-on till streaming is started.

And drop the deprecated s_power callback since this now no long
is necessary.

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>