drm/nouveau/mmu/tu102-: remove write to 0x100e68 during tlb invalidate

This was cargo-culted from traces of RM when the code was written, but
we probably shouldn't be touching NV_PFB regs while GSP-RM is running.

From traces, it looks like NVIDIA dropped this sometime between 510.54
and 515.48.07, so I guess we can too.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230918202149.4343-2-skeggsb@gmail.com