drm/vmwgfx: Extend the command verifier to handle guest-backed on / off

Make sure we disallow commands if the device doesn't support them.

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