[media] v4l2-subdev.h: add 'which' field for the enum structs

While all other pad ops allow you to select whether to use the 'try' or
the 'active' formats, the enum ops didn't have that option and always used
'try'.

However, this will fail if a simple (e.g. PCI) bridge driver wants to use
the enum pad op of a subdev that's also used in a complex platform driver
like the omap3. Such a bridge driver generally wants to enum formats based
on the active format.

So add a new 'which' field to these structs. Note that V4L2_SUBDEV_FORMAT_TRY
is 0, so the default remains TRY (applications need to set reserved to 0).

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Tested-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>