drm/i915: Properly track domain of the fbcon fb

X could end up putting the fbcon fb into other domains, e.g.
for smooth take-overs. Also we want this for accurate frontbuffer
tracking: The set_config is an implicit flush and will re-enable
psr and similar features, so we need to bring the bo back into
the gtt domain.

v2: Add FIXME comment about fbcon locking fun in atomic context,
requested by Chris.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>