drm: remove include of drmP.h from bridge/dw_hdmi.h

drmP.h is an relic from the days when there was a single header file.
To enable the removal of drmP.h from all users drop include
of drmP.h from bridge/dw_hdmi.h.

A few files relied on the file included in drmP.h - add explicit
include statements or forward declarations to these files.
Build tested with arm and x86.

v2:
- prefer forward declarations when possible (Laurent Pinchart)
- sort include files (Laurent Pinchart)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Archit Taneja <architt@codeaurora.org>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190108192939.15255-7-sam@ravnborg.org