r600/sfn: various fixes:
- Fix reporting TGSI IR support
- improve scheduling to give RA a better chance to succeed
- Handle color0 writes all on R700 like on EG
- Use low register number for unused write
Fixes:
- #6975 (closed)
- #6856 (closed)
- some hangs on R700
- regression that compute support would no longer be reported as supported when NIR was not set as preferred IR