media: rockchip: rga: split src and dst buffer setup

Split the register setup for the source and destination video buffers
into separate functions.

This is a cleanup to make the code more readable by separating the
offset calculation for the different buffers and prepares the driver for
using pre-calculated offsets of planes.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>