Calculate the plane offsets and store them with the video buffer while
creating the buffer mapping.

This allows the driver to more freely handle the memory of the DMA
mapping as the offsets and the mapping can be kept in sync.

The driver still has to update the offsets to respect the configured
cropping and rotation, but this calculation is now separated from the
calculation of the plane offsets.
Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>