drm/nve4/graph: do not crash if no power device present

Detect and workaround the absence of a power device so chips that do not
feature one (e.g. GK20A) can still use this driver.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>