drm/nouveau/fb: fix null derefs in nv49 and nv4e init

Commit dceef5d87 (drm/nouveau/fb: initialise vram controller as pfb
sub-object) moved some code around and introduced these null derefs.
pfb->ram is set to the new ram object outside of this ctor.

Reported-by: Ronald Uitermark <ronald645@gmail.com>
Tested-by: Ronald Uitermark <ronald645@gmail.com>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>