drm/vmwgfx: Don't kill clients on VT switch

DRI clients that tried to grab the TTM lock when the master (X server) was
switched away during a VT switch were sent the SIGTERM signal by the
kernel. Fix this so that they are only sent that signal when the master has
exited.

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