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

This reverts commit 8cb12a2528d795c45bba5f03b3486b4040fb0f45.

The commit fixed the OOB, yet it broke drmDevices2(0, NULL, 0) - aka we
did not return the total devices list.

Reviewed-by: Simon Ser <contact@emersion.fr>