Switch prison check from PROV_DRIVER to PRIV_KMEM_WRITE (like /dev/mem).

Access to the dri device gives effectively access to the entire memory of the machine (you can program
the graphic card to do DMA).

For current/stable/release this is a NOP, as access to memory is not allowed in a jail. This puts the dri
device into the same (in)security class than /dev/mem for future use.

Discussed with: anholt(?) several years ago
Sponsored by: Hackathon Essen 2017