modesetting: Set ppix->fb_id to 0 after removing the fb

This ensures the fb gets re-added when a shared pixmap is re-used for
a second drmmode_set_scanout_pixmap_cpu call.

Note currently the xserver never re-uses a shared pixmap in this way,
so this is mostly a sanity fix.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>

hw/xfree86/drivers/modesetting/drmmode_display.c

@@ -693,6 +693,7 @@ drmmode_set_scanout_pixmap_cpu(xf86CrtcPtr crtc, PixmapPtr ppix)
         if (crtc->randr_crtc->scanout_pixmap) {
             ppriv = msGetPixmapPriv(drmmode, crtc->randr_crtc->scanout_pixmap);
             drmModeRmFB(drmmode->fd, ppriv->fb_id);
+            ppriv->fb_id = 0;
         }
         if (drmmode_crtc->slave_damage) {
             DamageUnregister(drmmode_crtc->slave_damage);