anv: Add a helper to extract clear color from the attachment.

anv: Add a helper to extract clear color from the attachment.

Extract the code from color_attachment_compute_aux_usage, so we can
later reuse it to update the clear color state buffer.

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>