[media] davinci_vbpe: stop MEDIA_ENT_T_V4L2_SUBDEV abuse

This driver is abusing MEDIA_ENT_T_V4L2_SUBDEV:

- it uses a hack to check if the remote entity is a subdev;
- it still uses the legacy entity subtype check macro, that
  will be removed soon.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>