radv: don't emit no color formats. (v3)

radv: don't emit no color formats. (v3)

If we had no rasterization, we'd emit SPI color
format as all 0's the hw dislikes this, add the workaround
from radeonsi.

Found while debugging tessellation

v2: handle at pipeline stage, we have to handle
it after we process the fragment shader. (Bas)
v3: simplify even further, remove old fallback.

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