V4L/DVB (4065): Several improvements at videodev.c

Videodev now is capable of better handling V4L2 api, by
processing V4L2 ioctls and using callbacks to the driver.
The drivers should be migrated to the newer way and the older
one will be obsoleted soon.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>