drm: Fix an unwanted master inheritance v2

drm: Fix an unwanted master inheritance v2

commit a0af2e538c80f3e47f1d6ddf120a153ad909e8ad upstream.

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().

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
[bwh: Backported to 3.2:
 - s/master_mutex/struct_mutex/
 - drm_new_set_master() must drop struct_mutex while calling
   drm_driver::master_create
 - Adjust filename, context, indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>