V4L/DVB (4516): Split audio/video encoders/decoders from main drivers

Several audio/video encoders/decoders were just bound as if they were 
part of the core driver.
In fact, they are generic enough to be used by other drivers as well, 
since they use I2C event bus to communicate.

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