radv: Enable RB+ where possible.

radv: Enable RB+ where possible.

According to Marek, not enabling it on Stoney has a significant
negative performance impact. (And I guess this might impact
performance on Raven as well)

The register settings are pretty much copied from radeonsi. I did
not put this in the pipeline as that would make the pipeline more
dependent on the format which mean we would have to have more
pipelines for the meta shaders.

v2: Don't clear RB+ regs if not enabled as the CLEAR_STATE packet
    does already.
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>