drm/panel: st7703: 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/d7ebeec861f4518c8497a5e07d09d5a9fd123d3d.1633959458.git.agx@sigxcpu.org