radv: Handle VK_ATTACHMENT_UNUSED in CmdClearAttachment

radv: Handle VK_ATTACHMENT_UNUSED in CmdClearAttachment

From the Vulkan 1.0.98 spec for vkCmdClearAttachments:

"If any attachment to be cleared in the current subpass is VK_ATTACHMENT_UNUSED,
then the clear has no effect on that attachment."

"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."

"If the aspectMask member of any element of pAttachments contains
VK_IMAGE_ASPECT_DEPTH_BIT, then the current subpass' depth/stencil attachment
must either be VK_ATTACHMENT_UNUSED, or must have a depth component"

"If the aspectMask member of any element of pAttachments contains
VK_IMAGE_ASPECT_STENCIL_BIT, then the current subpass' depth/stencil attachment
must either be VK_ATTACHMENT_UNUSED, or must have a stencil component"

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>