drm/ingenic: Implement proper .atomic_get_input_bus_fmts

The .atomic_get_input_bus_fmts() callback of our top bridge should
return the possible input formats for a given output format. If the
requested output format is not supported, then NULL should be returned,
otherwise the bus format negociation will end with a bus format that the
encoder does not support.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/ab25925723cff2f3e773e7137567ef86fff5fdba.1649330171.git.hns@goldelico.com