media: vicodec: add struct for encoder/decoder instance

Add struct 'vicodec_dev_instance' for the fields in vicodec_dev
that have have both decoder and encoder versions.

Signed-off-by: Dafna Hirschfeld <dafna3@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>