drm/nv50-nv86,nv92/mc: rearm msi via pci config space, rather than mmio mirror

This is what NVIDIA do on these chipsets, let's hope it works around
the reported MSI failures for us on NV86.

v2: updated to include G92, as per information provided by NVIDIA.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>