radeonsi: avoid use of an uninitialized pipe_context in si_create_vertex_state()

Have si_create_vertex_elements_int() take si_screen* as that is what it needs.

Have the externally-exposed si_create_vertex_elements(pipe_context*) be an adapter calling into si_create_vertex_elements_int().

This avoids the need to create a fake pipe_context in si_create_vertex_state() just to pass the si_screen in.