The entity (device) has a locking SAREA and a master file descriptor
that optionally isn't closed between server generation.

The locking SAREA contains the device hardware lock.
Each DRI screen creates an new SAREA containing the drawable lock,
drawable-and private info, the drawable SAREA.

The first screen optionally shares its drawable SAREA with the
device SAREA.

Default is to close the master descriptor between server generations,
and to share the drawable SAREA of the first screen with the device locking
SAREA. Thus we should (hopefully) have full backwards compatibility.

Mesa changes to support single-device multiple screens are pending.