drm/vmwgfx: Allow dropped masters render-node like access on legacy nodes v2

Applications like gnome-shell may try to render after dropping master
privileges. Since the driver should now be safe against this scenario,
allow those applications to use their legacy node like a render node.

v2: Add missing return statement.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>