anv/blorp: Handle VK_ATTACHMENT_UNUSED in CmdClearAttachments

anv/blorp: Handle VK_ATTACHMENT_UNUSED in CmdClearAttachments

From the Vulkan 1.0.29 spec for vkCmdClearAttachments:

   "If the subpass’s depth/stencil attachment is VK_ATTACHMENT_UNUSED,
   then the clear has no effect."

and

   "If colorAttachment is VK_ATTACHMENT_UNUSED then the clear has no
   effect."

I have no idea why it's spec'd this way; it seems very anti-Vulkan to me,
but that's what it says and it's really not much work to support.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>