media: imx: imx-mipi-csis: Don't use .s_power()

The subdev .s_power() operation is deprecated. Drop it, requiring sensor
drivers to correctly use runtime PM instead of relying on .s_power().

As this driver has just been moved out of staging, and necessary drivers
to implement a full camera pipeline are still in staging, no platform
depends yet on this API being called. There is thus no risk of
regression.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>