radv: enable ACO support for GFX6

radv: enable ACO support for GFX6

CTS should pass, as well as Crucible and the few number of Piglit tests.

List of game benchmarks tested:
- Dawn of War 3
- Serious Sam 2017
- Shadow of The Tomb Raider
- The Talos Principle
- Thrones of Britannia
- Total Warhammer 2
- Total War: Three Kingdoms

Note that F12017 hangs with or without ACO on GFX6 at the moment.

My whole pipelinedb (~30 games) doesn't trigger any compiler crashes.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2401
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3533>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3533>