media: vsp1: drm: Split RPF format setting to separate function

The code that initializes the RPF format-related fields for display
pipelines will also be useful for the WPF to implement writeback
support. Split it from vsp1_du_atomic_update() to a new
vsp1_du_pipeline_set_rwpf_format() function.

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>