media: atomisp: Add atomisp_select_input() helper

When switching input/sensor the s_power() callback must be called
for old sensor drivers to power on the new sensor and power off
the previous sensor.

atomisp_s_input() already does this but atomisp_link_setup()
did not do this.

Add a new atomisp_select_input() helper which does this and use this
in both atomisp_s_input() and atomisp_link_setup() for consistent
behavior.

Also make atomisp_link_setup() turn the sensor back off when
a link gets disabled.

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>