fbdev: unify unlink_framebuffer paths

For some reasons the pm_vt_switch_unregister call was missing from the
direct unregister_framebuffer path. Fix this.

v2: fbinfo->dev is used to decided whether unlink_framebuffer has been
called already. I botched that in v1. Make this all clearer by
inlining __unlink_framebuffer.

v3: Fix typoe in subject (Maarten).

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
Cc: Peter Rosin <peda@axentia.se>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528090304.9388-20-daniel.vetter@ffwll.ch