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>

drivers/gpu/drm/i915/intel_display.c

@@ -6828,7 +6828,7 @@ intel_set_config_compute_mode_changes(struct drm_mode_set *set,
 			config->fb_changed = true;
 	}
 
-	if (set->x != set->crtc->x || set->y != set->crtc->y)
+	if (set->fb && (set->x != set->crtc->x || set->y != set->crtc->y))
 		config->fb_changed = true;
 
 	if (set->mode && !drm_mode_equal(set->mode, &set->crtc->mode)) {