drm/omap: fix bus_flags for panel-dpi

panel-dpi used to convey the bus-flags via the videomode, but recent
changes changed the use of videomode to DRM's drm_display_mode which
does not contain bus-flags. This broke panel-dpi, which didn't
explicitly store the bus-flags into dssdev->bus_flags.

Fix this by setting dssdev->bus_flags. Also change the bus_flags type to
u32, as that is the type used in the DRM framework, and we would get a
warning with drm_bus_flags_from_videomode() otherwise.

Fixes: 3fbda31e

 ("drm/omap: Split mode fixup and mode set from encoder enable")
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: H. Nikolaus Schaller <hns@goldelico.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181126092447.11864-1-tomi.valkeinen@ti.com
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>