worker: force KD_GRAPHICS mode before doing VT switch

If we switch VTs when in KD_TEXT mode there's obvious flicker.
This commit addresses that problem by going to KD_GRAPHICS mode
before switching VTs.  Ideally, we wouldn't switch VTs at all,
and instead leave it up to the display servers to manage via
logind. At the moment, the display servers don't use logind
properly, though, so do this as a stop gap.

https://bugzilla.gnome.org/show_bug.cgi?id=745031