drm/i915/chv: Turn off dclkp after the PLL has been disabled

During the enable sequence we first enable the dclkp output to the
display controller, and then enable the PLL. Do the opposite during
the disable sequence.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>