drm/radeon/kms: improve performance of blit-copy

[Ilija Hadzic]

On Thu, 13 Oct 2011, Roland Scheidegger wrote:

> I guess it isn't possible to temporarily disable some RBEs or otherwise
> reconfigure the chip that you could get the same performance for the
> high-end chips?

According to the conversation I had with Alex, this *is* possible but 
requires the pipeline and cache flush. So it is unclear what the overall 
gain will be given the flush penalty.

Also, this phenomena occurs only when GTT is involved in the copy. 
VRAM-to-VRAM copy in which there is no host memory involved (for which I 
added a benchmark, but didn't report in my note yesterday), high-end 
devices are beating low-end ones big time .... they better be ;-)

So if we can get RBE-reduction to work, it should be turned on only when 
one of the BOs is in GTT domain. I looked at what it would take to do 
this, and it's doable, but requires hacks at many places.

-- Ilija