drm/i915: don't update the fb base if there is no fb

Otherwise we'll set_fb complains pretty loudly if we the crtc is off
and userspace moves the NULL fb around a bit. Yeah, this actually
happens in the wild ...

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>