drm/i915: fix VDD override off wait

If we're disabling the VDD override bit and the panel is enabled, we
don't need to wait for anything. If the panel is disabled, then we
need to actually wait for panel_power_cycle_delay, not
panel_power_down_delay, because the power down delay was already
respected when we disabled the panel.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>