radv: Implement alternate GFX9 scissor workaround.
This improves dota2 performance for me by 11% when I force the GPU DPM level to low (otherwise dota2 is CPU limited for 4k on my threadripper), which should be a large part of the radv-amdvlk gap. (For me with that was radv 60.3 -> 66.6, while AMDVLK does about 68 fps) It looks like dota2 rendered the GUI with a bunch of draws with a SetScissors before almost each draw, causing a lot of pipeline stalls. I'm not really happy with the duplication of code, but overriding radeon_set_context_reg would also be messy since we have the pre-recorded pipelines and a bunch of si_cmd_buffer code, as well as some memory->context reg loads for which things would be more complicated. Reviewed-by: Dave Airlie <email@example.com> Reviewed-by: Samuel Pitoiset <firstname.lastname@example.org>