mesa/st: Handle 16-bit types at st_glsl_storage_type_size()

mesa/st: Handle 16-bit types at st_glsl_storage_type_size()

This is basically to avoid "not handle in switch" warnings.

v2: Let the new types hit the assertion instead. (Marek Olšák
    and Jason Ekstrand)

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>