[media] omap3/omap4/davinci: get rid of MEDIA_ENT_T_V4L2_SUBDEV abuse

On omap3/omap4/davinci drivers, MEDIA_ENT_T_V4L2_SUBDEV macro is
abused in order to "simplify" the pad checks.

Basically, it does a logical or of this macro, in order to check
for a local index and if the entity is either a subdev or not.

As we'll get rid of MEDIA_ENT_T_V4L2_SUBDEV macro, replace it by
2 << 16 where it occurs, and add a note saying that the code
there is actually a hack.

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