mesa: Generalize TexStorage allocator between swrast and intel.

mesa: Generalize TexStorage allocator between swrast and intel.

This should be reusable for other non-gallium drivers, so we can make the
extension always be available.

v2: Add a more detailed comment than the old function had (recommended
    by Brian).

Reviewed-by: Brian Paul <brianp@vmware.com> (v1)