With the new support for immediate vblank disabling we always disabled
the vblank interrupt right away, irrespective of the vblank offdelay
setting.

But being able to let vblanks run forever is fairly useful for
debugging, so restore that behaviour.
Suggested-by: NMario Kleiner <mario.kleiner.de@gmail.com>
Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NMatt Roper <matthew.d.roper@intel.com>
Reviewed-by: NMario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>