anv/blorp: Properly handle VK_ATTACHMENT_UNUSED

anv/blorp: Properly handle VK_ATTACHMENT_UNUSED

The Vulkan driver was originally written under the assumption that
VK_ATTACHMENT_UNUSED was basically just for depth-stencil attachments.
However, the way things fell together, VK_ATTACHMENT_UNUSED can be used
anywhere in the subpass description.  The blorp-based clear and resolve
code has a bunch of places where we walk lists of attachments and we
weren't handling VK_ATTACHMENT_UNUSED everywhere.  This commit should
fix all of them.

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
Cc: <mesa-stable@lists.freedesktop.org>