drm/vmwgfx: Allow prime fds in the surface reference ioctls

Allow prime fds and at the same time block legacy handles for render-nodes
in the surface reference ioctls. This means these ioctls can be used
directly from prime-aware clients, and that they can be called from
render-nodes.

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