lavd/v4l2: don't use avpriv_ prefix for internal functions

No need to keep the old symbols around until a major bump since lavd functions
with the avpriv_ prefix were never exposed.

Signed-off-by: James Almer <jamrial@gmail.com>
Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
