[media] V4L: soc_camera_platform: add helper functions to manage device instances

Add helper inline functions to correctly manage dynamic allocation and
freeing of platform devices. This avoids the ugly code to nullify
device objects.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>