drm/bridge: use bus flags in bridge timings

The DRM bus flags convey additional information on pixel data on
the bus. All current available bus flags might be of interest for
a bridge. Remove the sampling_edge field and use bus_flags.

In the case at hand a dumb VGA bridge needs a specific data enable
polarity (DRM_BUS_FLAG_DE_LOW).

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>