drm/mediatek: drop use of drmP.h

Drop use of the deprecated drmP.h header file.

While touching the include files divide them up in blocks
in the typical order:

\#include <linux/*>

\#include <video/*>

\#include <drm/*>

\#include ""

And sort the includes in the blocks
Add the necessary includes to fix build after removal of drmP.h

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Link: https://patchwork.freedesktop.org/patch/msgid/20190716064220.18157-20-sam@ravnborg.org