Add bus_formats and num_bus_formats fields and
drm_display_info_set_bus_formats helper function to specify the bus
formats supported by a given display.

This information can be used by display controller drivers to configure
the output interface appropriately (i.e. RGB565, RGB666 or RGB888 on raw
RGB or LVDS busses).
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Thierry Reding <treding@nvidia.com>