drm/bridge: nwl-dsi: Add atomic_get_input_bus_fmts

Components further up in the chain might ask us for supported formats.

Without this MEDIA_BUS_FMT_FIXED is assumed which then breaks display
output with mxsfb since it can't determine a proper bus format.

We handle the bus formats that correspond to the DSI formats the bridge
can potentially output (see chapter 13.6 of the i.MX 8MQ reference
manual) - which matches what xsfb can input.

Fixes: b776b0f0

 ("drm: mxsfb: Use bus_format from the nearest bridge if present")

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1712f2b952694fd4484dfd8576fbc5b4d7adf042.1633959458.git.agx@sigxcpu.org