drm/nouveau: populate master subdev pointer only when fully constructed

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

diff --cc drivers/gpu/drm/nouveau/core/core/subdev.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/device/base.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/include/subdev/fb.h
Simple merge