drm/i915: Opt out of vblank disable timer on >gen2

Now that the vblank races are plugged, we can opt out of using
the vblank disable timer and just let vblank interrupts get
disabled immediately when the last reference is dropped.

Gen2 is the exception since it has no hardware frame counter.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>