[media] adv7604: Add 16-bit read functions for CP and HDMI

16-bit registers are read through two 8-bit read operations. Factorize
that common pattern into 16-bit read accessors to simplify the code.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>