drm/shmobile: drop use of drmP.h

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

v2:
- Add additional include/forward to shmob_drm_crtc.h
  to avoid relying on indirect includes (Laurent)
- Add additional includes to shmob_drm_regs.h to make
  it self-contained
- Add additional includes to shmob_drm_plane.h to make
  it self-contained

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: linux-renesas-soc@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20190715090227.GA27652@ravnborg.org