drm/nouveau/pm/nv50: TPC[0x3] must be used for PGRAPH muxs on G80

I thought that using TPC[0x0] like for G84:GT215 was sufficient on G80,
but it's actually not the case. According to NVIDIA PerfKit on Windows,
we have to configure PGRAPH related muxs on TPC[0x3] for this chipset.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>