Extend the vsp1_du_atomic_flush() API with writeback support by adding
format, pitch and memory addresses of the writeback framebuffer.
Writeback completion is reported through the existing frame completion
callback with a new VSP1_DU_STATUS_WRITEBACK status flag.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>