ac/surface/gfx6: Don't force a tile index for fmask.

ac/surface/gfx6: Don't force a tile index for fmask.

The bpe of the fmask often differs from the bpe of the main
surface. On SI that means it has to get a different tile
index.

addrlib is capable of figuring this out itself, so just pass
-1 instead to let it know that it is not preset.

Fixes: 9bf3570fed0 "ac/surface/gfx6: compute FMASK together with the color surface"
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106511
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106499
Reviewed-by: Marek Olšák <marek.olsak@amd.com>