gen_table_fourcc: strip _MODIFIER suffix for INVALID

This is the only modifier printed with a "_MODIFIER" suffix. It
looks inconsistent when callers already print this word (e.g.
"modifier: INVALID_MODIFIER").

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Marius Vlad <marius.vlad@collabora.com>