[media] doc-rst: Fix license for the media books

The kAPI, v4l-drivers and dvb-drivers never used the
GNU FDL license. The addition of such license header were
just due to copy-and-paste. So, let's fix it.

As the media_kapi were part of device-drivers.tmp, it is
under GPL v2+.

The other two books is an agregation of files without any
license explicitly specified. So, they're all bound to the
Kernel's COPYING license. So, they're GPL v2 only.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>