drm/nouveau/kms: Move struct nouveau_framebuffer.vma to struct nouveau_fbdev

The vma field of struct nouveau_framebuffer is a special field for the
the accelerated fbdev console. Hence there's at most one single instance
for the active console. Moving it into struct nouveau_fbdev makes struct
nouveau_framebuffer slightly smaller and brings it closer to struct
drm_framebuffer.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>