nvc0: only allocate 2 registers less for gv100+

It is unclear to my why we have a +5 in there, but there is something funky
going on and we need to allocate more. On my TU17 +3 works without issues
as well though and it gives a slight performance improvement in
heavier benchmarks.

Signed-off-by: Karol Herbst <kherbst@redhat.com>
30 jobs for !6152 with nv_volta_max_reg
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