drmi/rcar-du: prepare for drmP.h removal from drm_modeset_helper.h

drmi/rcar-du: prepare for drmP.h removal from drm_modeset_helper.h

The use of drmP.h is discouraged and removal of it from
drm_modeset_helper.h caused rcar-du to fail to build.

This patch introduce the necessary fixes to prepare for the
drmP.h removal from drm_modeset_helper.h.

Build tested on arm x86 and arm allmodconfig.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190119084014.5355-6-sam@ravnborg.org