draw/i915: move hwfmt array to i915 specific struct

draw/i915: move hwfmt array to i915 specific struct

There's no point in bloating the vertex_info struct everywhere with
information that's only used by i915 in a single place. Let's explicitly
store the hwinfo when needed, instead of piggy-backing on vertex_info.

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23851>