radv: Implement alternate GFX9 scissor workaround.

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 <airlied@redhat.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>