drm: introduce bus_flags in drm_display_info

Introduce bus_flags to specify display bus properties like signal
polarities. This is useful for parallel display buses, e.g. to
specify the pixel clock or data enable polarity.

Suggested-by: Thierry Reding <thierry.reding@gmail.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Manfred Schlaegl <manfred.schlaegl@gmx.at>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Stefan Agner <stefan@agner.ch>