panel-dt: Add support for bus-flags

There was a placeholder for the bus_flags of type enum drm_bus_flags. The bus flags can be supplied by the similar enum display_flags. the display_flags are part of the videomode and can be converted to bus_flags by using the drm_bus_flags_from_videomode() function.

By using this in the panel-dt driver we get the possibility to use the polarity settings like hsync-active, vsync-active, de-active, pixelclk-active and syncclk-active.

BCS 746-000430