media: video-i2c: check if chip struct has set_power function

Not all future supported video chips will always have power management
support, and so it is important to check before calling set_power() is
defined.

Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Reviewed-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>