anv: Handle VK_ATTACHMENT_UNUSED in colorAttachment

anv: Handle VK_ATTACHMENT_UNUSED in colorAttachment

From the Vulkan 1.0.98 spec for vkCmdClearAttachments:

"If the aspectMask member of any element of pAttachments contains
VK_IMAGE_ASPECT_COLOR_BIT, then the colorAttachment member of that
element must either refer to a color attachment which is VK_ATTACHMENT_UNUSED,
or must be a valid color attachment."

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>