mesa: replace _mesa_update_stencil() with helper functions

mesa: replace _mesa_update_stencil() with helper functions

The idea is to remove the dependency on _mesa_update_state_locked,
so that st/mesa can skip it for stencil state updates, and then stop
setting _NEW_STENCIL in mesa/main if the driver is st/mesa.

The main motivation is to stop invoking _mesa_update_state_locked for
certain state groups.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>