drm/vmwgfx: Tighten security around surface sharing v2

If using legacy (non-prime) surface sharing, only allow surfaces
to be shared between clients with the same master. This will block
malicious clients from peeking at contents at surfaces from other
(possibly vt-switched) masters.

v2:
s/legacy_client/primary_client/

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