xf86drm: cap number of reported devices by drmGetDevice(2)

Do as the documentation says - when devices non NULL, cap the reported
devices to max_devices. Otherwise we risk out-of-bound access
for users of the API.

v2:
 - Fix this w/o breaking the API

v3:
 - Drop local variables, flip inverted conditional (Simon)

Issue: https://gitlab.freedesktop.org/mesa/drm/-/issues/56


Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>