drm/arcpgu: 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 arcgpu to fail to build.

This patch introduce the necessary fixes to prepare for the
drmP.h removal from drm_modeset_helper.h.
List of include files sorted alphabetically.

Build tested on arm x86 and arm allmodconfig.

Signed-off-by: Sam Ravnborg <[e-mail]>
Acked-by: Alexey Brodkin <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/arc/arcpgu_sim.c

···
  *
  */
 
-#include <drm/drm_crtc_helper.h>
 #include <drm/drm_atomic_helper.h>
+#include <drm/drm_crtc_helper.h>
+#include <drm/drm_device.h>
 
 #include "arcpgu.h"