iris: Add read-only domain for VF cache.

iris: Add read-only domain for VF cache.

This will allow us to order writes and reads of vertex and index
buffers by using the same cache tracking infrastructure introduced
previously for render and depth buffers.  The ultimate goal is to
remove the somewhat heavy-handed history flush mechanism currently
used for buffer objects, and use a single cache tracking mechanism
across the whole driver.

v2: Use C99 designated initializers (Ken).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12691>