media: imx355: Enable runtime PM before registering async sub-device

As the sensor device maybe accessible right after its async sub-device is
registered, such as ipu-bridge will try to power up sensor by sensor's
client device's runtime PM from the async notifier callback, if runtime PM
is not enabled, it will fail.

So runtime PM should be ready before its async sub-device is registered
and accessible by others.

Fixes: df0b5c4a

 ("media: add imx355 camera sensor driver")
Cc: stable@vger.kernel.org
Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>