anv: Use write_image_view to initialize immutable samplers

anv: Use write_image_view to initialize immutable samplers

Instead of setting it manually, call the helper.  When setting
descriptor sets becomes more complicated than just setting some struct
values, this will keep immutable sampler handling correct.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>