media: v4l2-mc: get rid of global pad indexes

Now that all drivers are using pad signal types, we can get
rid of the global static definition, as routes are stablished
using the pad signal type.

The tuner and IF-PLL pads are now used only by the tuner core,
so move the definitions to be there.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>