anv: Stop allowing non-zero clear colors in input attachments

anv: Stop allowing non-zero clear colors in input attachments

Previously, we bent over backwards to allow non-zero clear colors input
attachments whenever we could.  However, very few apps use input
attachments and very few use non-zero clear colors.  Getting rid of
support for non-zero clear colors input attachments will allow us to
treat them identically to textures which should help us simplify things
a good bit.

Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4393>