drm/nouveau/i2c: fix number of aux event slots

This was completely bogus before, using maximum DCB device index rather
than maximum AUX ID to size the buffer that stores event refcounts.

*Pretty* unlikely to have been an actual problem on most configurations,
that is, unless you've got one of the rare boards that have off-chip DP.

There, it'll likely crash.

Cc: stable@vger.kernel.org # 6.4+
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230719044051.6975-1-skeggsb@gmail.com