drm/bridge: make dw_mipi_dsi.h self-contained

To allow users to include dw_mipi_dsi.h without pulling in dependencies
make dw_mipi_dsi.h self-contained.
Use forward declarations when possible.

v2:
- Drop forward declarations of local structs (Laurent)
- Add include of drm_modes.h (Laurent)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Peter Senna Tschudin <peter.senna@gmail.com>
Cc: Martin Donnelly <martin.donnelly@ge.com>
Cc: Martyn Welch <martyn.welch@collabora.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190519183636.19588-2-sam@ravnborg.org