The entity .configure_partition() function operates on a partition, and
has to retrieve that partition from the pipeline's current partition
field. Pass the partition pointer to the function to make it clearer
what partition it operates on, and remove the vsp1_pipeline.partition
field.

This change clearly shows that the DRM pipeline doesn't use partitions,
which makes entity implementation more complex and error-prone. This
will be addressed in a further cleanup.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi+renesas@ideasonboard.com>