media: smiapp: Register sensor after enabling runtime PM on the device

Earlier it was possible that the parts of the driver that assumed runtime
PM was enabled were being called before runtime PM was enabled in the
driver's probe function. So enable runtime PM before registering the
sub-device.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>