rename and extend convert_idx_to_tuple

Support for more input values and function is renamed from _convert_idx_tuple
to _convert_idx_to_tuple.

Signed-off-by: Heiko Thiery <heiko.thiery@kontron.com>
add support for getting display string

For supporting this the get function is exported to _get().

Signed-off-by: Heiko Thiery <heiko.thiery@kontron.com>