drm: panel: Fix bus format for OrtusTech COM43H4M85ULC panel

The OrtusTech COM43H4M85ULC panel is a 18-bit RGB panel, set the bus
format to MEDIA_BUS_FMT_RGB666_1X18.

Fixes: 725c9d40 ("drm/panel: Add support for OrtusTech COM43H4M85ULC panel")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sam Ravnborg <sam.ravnborg@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200812220244.24500-1-laurent.pinchart@ideasonboard.com