drm/panel: mantix: Add media bus format

This allows the DSI bridge to detect the correct bus format.
We currently only support MEDIA_BUS_FMT_RGB888_1X24.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/75fbe7139a84fa133499afe242c204ba4516da98.1633959458.git.agx@sigxcpu.org