drm/stm: prepare for drmP.h removal from drm_modeset_helper.h

drm/stm: 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 drm/stm 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 and x86 allmodconfig

v2:
- sort list of include files

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Yannick Fertre <yannick.fertre@st.com>
Cc: Philippe Cornu <philippe.cornu@st.com>
Cc: Vincent Abriou <vincent.abriou@st.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/20190119084014.5355-2-sam@ravnborg.org