drm/vmwgfx: Hook up guest-backed contexts

Contexts are managed by the kernel only, so disable access to GB
context commands from user-space

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