[media] cec: Add cec_get_drvdata()

Add a helper function to get driver private data from CEC
adapter. This helps the readability a little bit and allows
to change the 'priv' field name to something else without
needing to touch all drivers.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>