drm/nouveau: Use generic helper to check _PR3 presence

Replace nouveau_pr3_present() in favor of a more generic one,
pci_pr3_present().

Also the presence of upstream bridge _PR3 doesn't need to go hand in
hand with device's _DSM, so check _PR3 before _DSM.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>