drm/stm: drop use of drmP.h

Drop use of the deprecated header file drmP.h
from the sole user in the stm driver.
Replace with necessary include files.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Yannick Fertre <yannick.fertre@st.com>
Cc: Philippe Cornu <philippe.cornu@st.com>
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>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-stm32@st-md-mailman.stormreply.com
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190630061922.7254-4-sam@ravnborg.org