drm/fsl-dcu: drop use of drmP.h

Drop use of the deprecated drmP.h header file.
Fix fallout.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Stefan Agner <stefan@agner.ch>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Alison Wang <alison.wang@nxp.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190630061922.7254-9-sam@ravnborg.org