drm/sti: drop use of drmP.h

Stop using the deprecated drmP.h header file.
Replaced with relevant forwards or headers files.
Header files sorted in all files touched.

Build tested with allyesconfig, allmodconfig for a number of
architectures.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190605134835.25112-2-sam@ravnborg.org