If we have an unused CRTC or connector, explicitly disable it during the
end of the repaint cycle, or when we get VT-switched back in.

This commit moves state_invalid from an output property to a backend
property, as the unused CRTCs or connectors are likely not tracked by
drm_outputs. This matches the mechanics of later commits, where we move
to a global repaint-flush hook, applying the state for all outputs in
one go.

The output state_invalid flag originally provoked full changes on output
creation (via setting the flag at output enable time) and session enter.

For the new-output case, we will not have any FB in output->state_cur,
so we still take the same path in repaint as if state_invalid were set.
At session enter, we preserve the existing behaviour: as
start_repaint_loop will fail when state_invalid is set, all outputs will
be scheduled for repaint together, and state_invalid will not be cleared
until after all outputs have been repainted, inside repaint_flush.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>