freedreno/ir3: Fix assertion failures dumping CS high full regs.
The *2 here would bump into the *2 in regset, causing assertion failures
dumping CS programs. Just set the mergedregs flag on a6xx, and don't
duplicate the mergedregs logic. If you're dealing with new HW where we
don't know if mergedregs is set, you may need to tweak the flag during
disasm setup for the stats to make sense.
Fixes: f7bd3456d79a ("freedreno: deduplicate a3xx+ disasm")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6323>