drm/i915: Turn force VDD back off when panel running in intel_dp_dpms

The VDD force bit is turned on before touching the panel, but if it
was enabled, there was no call to turn it back off. Add a call.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>