drm: Fix an unwanted master inheritance v2

A client calling drmSetMaster() using a file descriptor that was opened
when another client was master would inherit the latter client's master
object and all its authenticated clients.

This is unwanted behaviour, and when this happens, instead allocate a
brand new master object for the client calling drmSetMaster().

Fixes a BUG() throw in vmw_master_set().

Cc: <stable@vger.kernel.org>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>