drm: rcar-du: vsp: Extract framebuffer (un)mapping to separate functions

The rcar_du_vsp_plane_prepare_fb() and rcar_du_vsp_plane_cleanup_fb()
functions implement the DRM plane .prepare_fb() and .cleanup_fb()
operations. They map and unmap the framebuffer to/from the VSP
internally, which will be useful to implement writeback support. Split
the mapping and unmapping out to separate functions.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>