st/mesa: consolidate view format setup code

st/mesa: consolidate view format setup code

Before, we had code to compute the sampler view's format spread across two
different functions: in update_single_texture() and
st_get_texture_sampler_view_from_stobj().  Now it's all in one new function.

Also, use _mesa_texture_base_format() to simplify the code.

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>