xf86drmMode.h: use ANSI C99 arrays

This avoids the use of a GNU-specific extension in public headers. Also
see [1].

[1]: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/80#note_707458



Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>