ac: rename SI-CIK-VI to GFX6-GFX7-GFX8

ac: rename SI-CIK-VI to GFX6-GFX7-GFX8

Acked-by: Dave Airlie <airlied@redhat.com>
We already use GFX9 and I don't want us to have confusing naming
in the driver. GFXn naming is better from the driver perspective,
because it's the real version of the gfx portion of the hw. Also,
CIK means Bonaire-Kaveri-Kabini, it doesn't mean CI.

It shouldn't confuse our SDMA, UVD, VCE etc. code much. Those have
nothing to do with GFXn and they have their own version numbers.