drm/vmwgfx: Reinstate and tighten security around legacy master model

The following restrictions affect clients connecting using legacy nodes:

*) Masters that have dropped master privilieges are not considered
   authenticated until they regain master privileges.
*) Clients whose master have dropped master privileges block interruptibly on
   ioctls  requiring authentication until their master regains master
   privileges. If their master exits, they are killed.

This is primarily designed to prevent clients authenticated with one master to
access data from clients authenticated with another master.
(Think fast user-switching or data sniffers enabled while X is vt-switched).

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>