drm/nouveau/pwr: make nouveau_pwr_pgob() non-static

The platform device does not use the common nouveau_pwr_init() to initialize
the PWR, but it does need the .pgob() be assigned to avoid NULL pointer
dereference in graph/nve4.c.

Signed-off-by: Vince Hsu <vinceh@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>