nir-to-tgsi: fix hw atomic counter offset

The atomic counter offset is already mapped into the memory.Index field.
So the offset in the TGSI atomic instruction should be zero.

Fixes spec@arb_compute_shader@execution@atomic-counter
      spec@arb_compute_shader@execution@basic-*  with SVGA driver
193 jobs for !16459 with fix-ntt-atomic-counter-offset in 31 minutes and 28 seconds (queued for 9 seconds)
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