anv/pass: Store usage in each subpass attachment

anv/pass: Store usage in each subpass attachment

This requires us to ditch the VkAttachmentReference struct in favor of
an anv-specific struct.  However, we can now easily identify from just
the subpass attachment what kind of an attachment it is.  This will make
iteration over anv_subpass::attachments a little easier in some case.

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>