[media] smiapp: Initialise media entity after sensor init

This allows determining the number of pads in the entity based on the
sensor.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>