toradex: tdx-cfg-block: rework display adapter name handling

Rework the rather big array of zero length strings with 4 entries of
actual display adapter names to a array of structs which ties a pid4
to its correspondent human readable string.
Provide an accessor to get the string for a given PID4.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Andrejs Cainikovs <andrejs.cainikovs@toradex.com>