[media] media: platform: pxa_camera: add missing sensor power on

During sensors binding, there is a window where the sensor is switched
off, while there is a call it to set a new format, which can end up in
an access to the sensor, especially an I2C based sensor.

Remove this window by activating the sensor.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>