radeonsi: rework RB+ for Stoney

radeonsi: rework RB+ for Stoney

This fixes it.

States which also need to be taken into account:
- SPI color formats - each down-conversion format supports only a limited set
  of SPI formats
- whether MSAA resolving and logic op are enabled

These need special handling:
- blending
- disabled channels

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>