radv: predicate cmask eliminate when using DCC.

radv: predicate cmask eliminate when using DCC.

When using DCC some clear values don't require a cmask eliminate
step. This patch adds support for black and black with alpha 1,
there are other values, but I don't have access to a comprehensive list.

This works by setting the cmask eliminate predicate when doing the
fast clear, and later when doing the cmask elimination making sure
the draws are predicated.

This increases the fps on Sascha Willems deferred.

Tonga: 580fps->670fps on a Tonga PRO card.
Polaris 730->850fps

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>