drm/i915: Initialize PPS timestamps on vlv/chv

The pps timestamp initialization was accidentally lost on vlv/chv in

 commit a4a5d2f8
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Thu Sep 4 14:54:20 2014 +0300

    drm/i915: Track which port is using which pipe's power sequencer

Restore it so that we avoid introducing random delays into the pps operations
during/after driver init time.

Cc: Imre Deak <imre.deak@intel.com>
Reported-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>