media: atomisp-ov2680: Save/restore exposure and gain over sensor power-down

Save/restore exposure and gain over sensor power-down and don't write them
to the sensor when ov2680_set_exposure() is called while the sensor is off.

Link: https://lore.kernel.org/linux-media/20211107171549.267583-7-hdegoede@redhat.com


Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>